alpha = "abcdefghijklmnopqrstuvwxyz"

def letter2index(c):
    for i in range(26):
        if c == alpha[i]:
            return(i)

def index2letter(n):
    return(alpha[n])

def caesar(s, key):
    n = len(s)
    t = ""
    for i in range(n):
        c = s[i]
        if c == " ":
            t = t + " "
        else:
            m = letter2index(c)
            encm = (m + key) % 26
            newc = index2letter(encm)
            t = t + newc
    return(t)