func compress(input: String) -> String {
    var input = input
    var count = 1
    var output = ""
    for char in input {
        if let last = output.last, last == char {
            count += 1
        }else {
            if count > 1 {
                output = output + String(count)
                count = 1
            }
            output = output + String(char)
        }
    }
    if count > 1 {
        output = output + String(count)
    }
    return output
}