C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	char r = '\0';
	string s;
	cin>>s;
	
	for(int i = 0;i < s.size();i++){
		if(s[i]%2==0 && s[i] > r){
			r = s[i];
		}
	}
	
	if(r == '\0') cout<<"No";
	else cout<<r;
	return 0;
}