#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <vector>
#include <stack>
#include <map>
#include <set>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <bitset>
#include <assert.h>
#include <unordered_map>

using namespace std;
typedef long long ll;
typedef set<int>::iterator ssii;

#define Cmp(a, b) memcmp(a, b, sizeof(b))
#define Cpy(a, b) memcpy(a, b, sizeof(b))
#define Set(a, v) memset(a, v, sizeof(a))
#define debug(x) cout << #x << ": " << x << endl
#define _forS(i, l, r) for(set<int>::iterator i = (l); i != (r); i++)
#define _rep(i, l, r) for(int i = (l); i <= (r); i++)
#define _for(i, l, r) for(int i = (l); i < (r); i++)
#define _forDown(i, l, r) for(int i = (l); i >= r; i--)
#define debug_(ch, i) printf(#ch"[%d]: %d\n", i, ch[i])
#define debug_m(mp, p) printf(#mp"[%d]: %d\n", p->first, p->second)
#define debugS(str) cout << "dbg: " << str << endl;
#define debugArr(arr, x, y) _for(i, 0, x) { _for(j, 0, y) printf("%c", arr[i][j]); printf("\n"); }
#define _forPlus(i, l, d, r) for(int i = (l); i + d < (r); i++)
#define lowbit(i) (i & (-i))
#define MPR(a, b) make_pair(a, b)

pair<int, int> crack(int n) {
    int st = sqrt(n);
    int fac = n / st;
    while (n % st) {
        st += 1;
        fac = n / st;
    }

    return make_pair(st, fac);
}

inline ll qpow(ll a, int n) {
    ll ans = 1;
    for(; n; n >>= 1) {
        if(n & 1) ans *= 1ll * a;
        a *= a;
    }
    return ans;
}

template <class T>
inline bool chmax(T& a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

ll gcd(ll a, ll b) {
    return b == 0 ? a : gcd(b, a % b);
}

ll ksc(ll a, ll b, ll mod) {
    ll ans = 0;
    for(; b; b >>= 1) {
        if (b & 1) ans = (ans + a) % mod;
        a = (a * 2) % mod;
    }
    return ans;
}

ll ksm(ll a, ll b, ll mod) {
    ll ans = 1 % mod;
    a %= mod;

    for(; b; b >>= 1) {
        if (b & 1) ans = ksc(ans, a, mod);
        a = ksc(a, a, mod);
    }

    return ans;
}

template <class T>
inline bool chmin(T& a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}

bool _check(int x) {
    //
    return true;
}

int bsearch1(int l, int r) {
    while (l < r) {
        int mid = (l + r) >> 1;
        if(_check(mid)) r = mid;
        else l = mid + 1;
    }
    return l;
}

int bsearch2(int l, int r) {
    while (l < r) {
        int mid = (l + r + 1) >> 1;
        if(_check(mid)) l = mid;
        else r = mid - 1;
    }
    return l;
}

template<class T>
bool lexSmaller(vector<T> a, vector<T> b) {
    int n = a.size(), m = b.size();
    int i;
    for(i = 0; i < n && i < m; i++) {
        if (a[i] < b[i]) return true;
        else if (b[i] < a[i]) return false;
    }
    return (i == n && i < m);
}

// ============================================================== //

const int maxn = 1000 + 10;
int C, n;

class A {
public:
    int x, y;
} a[maxn];

map<int, int> got;
vector<int> nums;
int sum[maxn][maxn];

// discrete
void discrete() {
    sort(nums.begin(), nums.end());
    nums.erase(unique(nums.begin(), nums.end()), nums.end());
    for (auto x : nums) got[x] = lower_bound(nums.begin(), nums.end(), x) - nums.begin();
}

// prework
void prework() {
    _rep(i, 1, n) {
        int x = got[a[i].x], y = got[a[i].y];
        sum[x][y]++;
    }

    _for(i, 1, nums.size()) _for(j, 1, nums.size()) {
        sum[i][j] += sum[i-1][j] + sum[i][j-1] - sum[i-1][j-1];
    }
}

// check
inline bool check(int R) {
    for (int x2 = 1, x1 = 0; x2 < nums.size(); x2++) {
        while (nums[x2] - nums[x1+1] + 1 > R) x1++;
        for(int y2 = 1, y1 = 0; y2 < nums.size(); y2++) {
            while (nums[y2] - nums[y1+1] + 1 > R) y1++;
            int cnt = sum[x2][y2] - sum[x1][y2] - sum[x2][y1] + sum[x1][y1];
            if (cnt >= C) return true;
        }
    }
    return false;
}

void solve() {
    int l = 1, r = 10000;
    while (l < r) {
        int mid = (l + r) >> 1;
        if (check(mid)) r = mid;
        else l = mid + 1;
    }
    printf("%d\n", r);
}

void init() {
    got.clear();
    nums.clear();
    nums.push_back(0);

    memset(sum, 0, sizeof sum);
}

int main() {
    //freopen("input.txt", "r", stdin);
    init();

    // get data
    scanf("%d%d", &C, &n);
    _rep(i, 1, n) {
        int x, y;
        scanf("%d%d", &x, &y);
        nums.push_back(x), nums.push_back(y);
        a[i].x = x, a[i].y = y;
    }

    // discrete
    discrete();

    // prework
    prework();

    // check and binary search
    solve();
}