#include<iostream> 
#include<cstdio> 
#include<cmath> 
#include<algorithm> 
#include<cstring> 
#include<string> 
using namespace std;
int main(){
	int y,m;
	cin >> y >> m;
	bool run;//闰年
	if(y%4 == 0) {
		if(y%100==0){
			if(y%400 == 0){
				run = 1;
			}else{
				run = 0;
			}
		}else{
			run = 1;
		}
	}
	if(m == 1||m == 3||m == 5||m == 7||m == 8||m == 10||m == 12){
		cout << 31;
	}
	if(m == 4||m == 6||m == 9||m == 11){
		cout << 30;
	}
	if(m == 2){
		if(run == 1){
			cout << 29;
		}else{
			cout << 28;
		}
	}
	return 0;
}

有壮壮，好评

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n,b;
    cin>>n>>b;
    if (b == 1 || b == 3 || b == 5 || b == 7 || b == 8 || b == 10 || b == 12)
    {
        cout<<31;
    }
    if (b == 4 || b == 6 || b == 9 || b == 11)
    {
        cout<<30;
    }
    if (b == 2)
    {
        if (n % 100 == 0)
        {
            if (n % 400 == 0)
            {
                cout<<29;
            }
            else
            {
                cout<<28;
            }
        }
        else if (n % 4 == 0)
        {
            cout<<29;
        }
        else
        {
            cout<<28;
        }
    }
    return 0;
}