#include <bits/stdc++.h> using namespace std; int main() { string S; cin >> S; stringstream ss; set<char> s; for (char c : S) { if (s.find(c) != s.end()) { ss << c << endl; break; } else { s.insert(c); } } string output; if (ss >> output) { cout << output << endl; } else { cout << "None" << endl; } }

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