#include <iostream>
#include <string>
#include <set>
using namespace std;
int main() {
string s;
cin >> s;
char c, k;
int last, now=1;
set <char> a;
for (int i=0; i<s.size(); i++) { //этот код проще по памяти и бытрее работает?
last = a.size();
now = last;
k = s[i];
a.insert(k);
now = a.size();
if (last == now) {
c = s[i];
}
}
cout << c;
return 0;
}
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