562 B | 562 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():
key = input()
n = len(key)
s = input()
ans = ""
j = 0
for i in s:
if 'A' <= i <= 'Z':
k = ord(key[j % n]) - 65
ans += chr((ord(i) - 65 + k) % 26 + 65)
j += 1
p(ans)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
solve()