#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 210;

int m, s, c;
int a[N], b[N];

int main()
{
    cin >> m >> s >> c;
    for (int i = 1; i <= c; i++) cin >> a[i];//牛所占牛棚的编号
    sort(a + 1, a + c + 1);
    for (int i = 2; i <= c; i++) b[i - 1] = a[i] - a[i - 1] - 1;//相邻牛之间的牛棚数
    sort(b + 1, b + c);
    int ans = c;//先每一牛一块要木板
    if(m < c){//如果木板小于牛数
        for (int i = 1; i <= c - m; i++){
            if(b[i] > 0)
            ans += b[i];//加空档
        }
    }
    cout << ans << endl;
    return 0;
}