#include<iostream>
#include<cmath>
using namespace std;
int main()
{
int y, m;
cin >> y >> m;
if(m == 4 || m == 6 || m == 9 || m == 11)
cout << 30;
else if(m == 2)
{
if(y % 4 == 0 && !(y % 100 == 0 && y % 400 != 0))
cout << 29;
else
cout << 28;
}
else
cout << 31;
return 0;
}

有标志认证，属三无产品，请大家放心食用

#include<iostream>
#include<cmath>
using namespace std;
int main()
{
	int y, m;
	cin >> y >> m;
	if(m == 4 || m == 6 || m == 9 || m == 11)
		cout << 30;
	else if(m == 2)
	{
		if(y % 4 == 0 && !(y % 100 == 0 && y % 400 != 0))
			cout << 29;
		else
			cout << 28;
	}
	else
		cout << 31;
	return 0;
}