答案算出来是错的

#include <iostream>
using namespace std;

int main() {
	int l, m, s = 0;
	cin >> l >> m;
	int tree[10001];
	for (int y = 0; y <= l; y++) {
		tree[y] = 1;
	}
	for (int i = 0; i <= m - 1; i++) {
		int front, tail;
		cin >> front >> tail;
		for (int x = front; x <= tail; x++) {
                tree[x] = 0;
		}
	}
	for (int a = 0; a <= l; a++) {
		if (tree[a] == 0) {
			s++;
		}
	}
	cout << s;
	return 0;
}