题解 | 校门外的树

#include <iostream>
#include<vector>
using namespace std;

int main() {
   int L,M;
   cin >> L >> M;
   int count = 0;
   vector<int>tree(10001,1);
   int l,r;
   while(M--){
    cin >> l >> r;
    for(int i = l;i<=r;i++){
        tree[i]=0;
    }
   }
   for(int i = 0;i<=L;i++){
       if(tree[i]){
        count++;
       }
   }
   cout << count << endl;
}