while(line = readline()) {
    const arr = line.split('');
    let result = arr.reduce((temp, data) => {
       temp[data]  = temp[data]  ? temp[data] + 1 : 1;
        return temp;
    }, {})
    let status = false;
    for (let index in result) {
        if (result[index] == 1) {
            status = true;
            print(index);
            break;
        }
    }
    if (!status) {
        print(-1);
    }
}