差分
#include <iostream>
#include <algorithm>
using namespace std;
typedef pair<int, int> PII;
const int N = 10010;
int n, m;
int f[N];
int main() {
cin >> n >> m;
int l, r;
while (m --) {
cin >> l >> r;
f[l] --;
f[r + 1] ++;
}
for (int i = 1; i <= n; i ++)f[i]+=f[i-1];
int res=0;
for (int i = 0; i <= n; i ++) {
res += (f[i]==0);
}
cout << res;
return 0;
}