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这题狗都会
↓以下为正文

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

int main() {
int L, M;
cin >> L >> M;

vector<int> diff(L + 2, 0);


for (int i = 0; i < M; i++) {
    int u, v;
    cin >> u >> v;
    diff[u] += 1;
    diff[v + 1] -= 1;
}


for (int i = 1; i <= L; i++) {
    diff[i] += diff[i - 1];
}


int remaining_trees = 0;
for (int i = 0; i <= L; i++) {
    if (diff[i] == 0) {
        remaining_trees++;
    }
}

cout << remaining_trees << endl;
return 0;
}

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