#include <iostream>
int main()
{
	using namespace std;
	int a[10003], b, m, f = 0;
	cin >> b >> m;

	for (int i = 0; i < b; i++)
		a[i] = 1;

	int A, B;
	for (m; m > 0; m--)
	{
		cin >> A >> B;
		for (A; A <= B; A++)
			a[A] = 0;
	}

	for (int i = 0; i < b; i++)
		f += a[i];

	cout << f;
	return 0;
}

我的代码比预料结果少1，请问是哪个步骤出现问题？