水

#include <bits/stdc++.h>
using namespace std;

int l, t;
int a[100010];
int ans;

int main(){
    cin >> l >> t;
    while (t--){
        int l1, r1;
        cin >> l1 >> r1;
        for (int i=l1;i<=r1;i++){
            a[i] = 1;
        }
    }

    for (int i=0;i<=l;i++){
        if (a[i]==0) ans++;
    }

    cout << ans;

    return 0;
}