为什么前三个通不过？？

#include<iostream>
using namespace std;

int main()
{
	int arr[100000] = {};
	int l, m,u, v,a=0;
	cin >> l >> m;
	for (int i = 0; i < m; i++)
	{
		cin >> u >> v;
		for (int j = u; j <= v; j++)
		{
			arr[j] = 1;
		}
	}
	for (int x = 0; x < l; x++)
	{
		a += arr[x];
	}
	cout << l-a+1;
}