#include <bits/stdc++.h>
using namespace std;
int main(){
    int l, m, a[10001] = {0};
    int r, le;
    cin >> l >> m;
    for(int i = 1; i <= m; i++){
        cin >> r >> le;
        for(int j = r; j <= le; j++){
            a[j] = 1;
        }
    }
    int cnt = 0;
    for(int i = 0; i <= l; i++){
        if(a[i] == 0)
            cnt++;
    }
    cout << cnt << endl;
    return 0;
}