AC with no WA

#include <iostream>
using namespace std;
int a[10001];
int main() {
    int N,M, L,R;
    cin >> N >> M;
    while (M--){
        cin >> L >> R;
        for (int i = L; i <= R; i++)
            a[i] = 1;
    }
    int cnt = 0;
    for (int i = 0; i <= N; i++)
        if (a[i] == 0)
            cnt ++;
    cout << cnt;
}