Various approaches are possible, among which the condition of being No is probably the easiest.

The answer is No if and only if

• there exist \((i, j)\) such that \(S_i = S_j (i \neq j)\).

This way, we can implement it as follows.

Sample code in C++:

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

signed main(){
    vector<string>s(4);
    for(int i=0;i<=3;i++)cin>>s[i];
    for(int i=0;i<=3;i++){
        for(int j=0;j<=3;j++){
            if(i!=j && s[i]==s[j]){
                cout<<"No"<<endl;
                return 0;  // terminate the program
            }
        }
    }
    cout<<"Yes"<<endl;
    return 0;
}