C++ :

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

int main(){
    int l, m;
    cin >> l >> m;
    int trees[10005] = {0};
    for(int i = 0; i < m; ++i){
        int s,e;
        cin >> s >> e;
        for(int j = s; j <= e; ++j){
            trees[j] = 1;
        }
    }

    int ans = 0;
    for(int i = 0; i <= l; ++i){
        if(trees[i] == 0){
            ans++;
        }
    }
    cout << ans << endl;
    return 0;
}