s2 = ''
    for ch in s :
        if ch.isalpha():
            t = ord(ch)
            if 96 <= t <= 122 :
                t += k
                while t > 122 :
                    t -= 26
            elif 65 <= t <= 90 :
                t += k 
                while t > 90 :
                    t -= 26
            s2 += chr(t)
        else :
            s2 += ch
    return s2