大佬看看哪里错了

#include <iostream>
using namespace std;
int main(void) {
	int arr[10010] = {}, lenth, many, start, end, count = 0;
	cin >> lenth >> many;
	for (int i = 0; i < lenth; i++)
	{
		arr[i] = 1;
	}
	for (int i = 0; i < many; i++)
	{
		cin >> start >> end;
		for (int j = start; j <= end; j++)
		{
			arr[j] = 0;
		}
	}
	for (int i = 0; i < many; i++)
	{
		if (arr[i] == 0) {
			count++;
		}
	}

	cout << count << endl;
	return 0;
}

一直输出为0