前面2个WA，后8个AC（个人猜测是没重合的情况没有触发

#include <bits/stdc++.h>

using namespace std;

int main() {
    int l,m,is;
    int u[m], v[m], isBigger[m-1];
    cin >> l >> m;
    for (int i=0; i<m; i++) {
        cin >> u[i] >> v[i];
    }
    
    sort(u, u+m);
    sort(v, v+m);
    
    for (int i=0; i<m; i++) {
        if (i>1 and u[i]>v[i-1]) {
            isBigger[i-1]=1;
        } else {
            isBigger[i-1]=0;
        }
    }
    
    sort(isBigger, isBigger+m-1);
    
    
    if (isBigger[0] == isBigger[m-2] and isBigger[0]==1) {
        for (int i=0; i<m; i++) {
            l-=v[i] - u[i] + 1;
            cout << l-1;
        }
    } else {
        cout << l-v[m-1]+u[0];
    }
}