614 B | 614 chars
import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
k = "ABCDEFGHIJKLMNOPQRSTUVWXYZ_."
while 1:
s = input()
if s == "0":
break
n, s = s.split()
n = int(n)
ans = ""
for i in s:
ans += k[(k.index(i) + n) % len(k)]
print(ans[::-1])
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()