题解 | 校门外的树

#include<bits/stdc++.h>
using namespace std;
int main(){
    int L,M,sum = 0;
    cin>>L>>M;
    vector<int> removed(L+1,0);
    for(int i = 0;i<M;i++){
        int a,b;
        cin>>a>>b;
        for(int j = a;j<=b;j++){
            removed[j] = 1;
        }
    }
    for(int i = 0;i<=L;i++){
        sum+=removed[i];
    }
    cout<<L+1-sum;
}