import * as fs from 'fs';
const input = fs.readFileSync('/dev/stdin', 'utf8');

const array = input.split('');
const number_array = array.map(Number);

let sum = 0;
for (let i = 1; i < number_array.length - 1; i++) {
    if (number_array[i] === number_array[i + 1]) {
        sum = sum + 1;
    }
}
if (sum === number_array.length - 2) {
    console.log(number_array[2]);
} else {
    console.log('-1');
}