function main(input){
    var alpha = [];
    for(var j=97;j<= 122;j++){
        alpha.push(String.fromCharCode(j));
    }
    var str = input.split("");
    var key;
    for(var i=0; i<str.length; i++){
        key = alpha.indexOf(str[i]);
        if(key>=0)
            alpha.splice(key,1);
    }

    if(alpha.length>0)
        console.log(alpha[0]);
    else
        console.log("None");
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));