val int s 3; // global

val [int->int] fib (int n) {
    return if eq n 0 {
        1
    } else {
        mul n fib 
    };
};

val [->int] main {
    val str line input;
    val int n to_int line;
    if less n 0 {
        print "illegal number"
    } else {
        print to_str {
            fib n
        };
    }
};