#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    string word;
    cin >> word;
    vector<int> v(word.size(), 0);
    for(int i = 0; i < word.size(); ++i) {
        for(int j = 0; j < word.size(); ++j) {
            if(word[i] == word[j]) {
                v[i]++;
            }
        }
    }
    cout << word.substr(max_element(v.begin(), v.end()) - v.begin(), 1);
}

Why does it work?