请问我哪里错了？

#include<iostream>
#include<cstring>

using namespace std;

int main()
//校门外的树
{
	int answer=0;
	int L, M;
	cin >> L >> M;
	int trees[10010];
	memset(trees, 0, 10010);
	for (int i = 1; i <= M; i++)
	{
		int head, tail;
		cin >> head >> tail;
		for (int j = head; j <= tail; j++)
		{
			if (trees[j]==0)
			{
				trees[j] = 1;
			}
		}
	}
	for (int i = 0; i <= L; i++)
	{
		if (trees[i] == 0)answer++;
	}
	cout << answer;
}