process.stdin.resume();
process.stdin.setEncoding('utf8');

var g_input = '';
process.stdin.on('data', function(chunk) {
    g_input += chunk;
});

process.stdin.on('end', function() {
    Main(g_input.split(/\r?\n/));
});

function Main(input) {
    var characters = input[0].split('').sort().filter((element, index, array) => array.indexOf(element) === index);
    var c = '';
    var search = 'a';

    for (c of characters) {

        if (c === search) {
            search = String.fromCharCode(search.charCodeAt() + 1);
        }
        else {
            console.log(search);
            return;
        }
    }

    if (search.charCodeAt() > 'z'.charCodeAt()) {
        console.log('None');
    }
    else {
        console.log(search);
    }
}
