const n = require("fs").readFileSync(0,"utf8").toString().toUpperCase();
let arr = new Array(26).fill(0);

for(let c of n){
    arr[c.charCodeAt()-'A'.charCodeAt()]++;
}

let max_idx = 0, cnt = 0;

for(let i=0; i<26; i++){
    if(arr[i]==Math.max(...arr)){
        max_idx=i;
        cnt++;
    }
}

console.log(cnt>1 ? '?' : String.fromCharCode('A'.charCodeAt()+max_idx));