#include <iostream>
#include <cstring>
#include <cstdio>
#include <cstdlib>
using namespace std;
int main(){
string str;
int a = 0;
while(getline(cin,str)){
for(int i = 0;i < str.size();++i){
if(str.find(str[i]) == str.rfind(str[i])){
a = 1;
cout << str[i];
break;
}
}
}
if(a == 0) puts("no");
return 0;
}