题解来啦！

include <bits/stdc++.h>
using namespace std;
const int N = 100001;
int n,a[N];
string s;
int main(){
cin>>s;
for(auto i:s){
a[ i-'a'] ++;
}
for(int i=0;i<=26;++i){
if(a[i]==1){
cout<<char(i+'a');
return 0;
}
}
cout<<"no";
return 0;
}