#!/usr/bin/env python3
str1 = 'abcd1234'
str2 = 'bcde2345'
str3 = '7:69h5i>'

ord1 = (ord(c) for c in str1)
ord2 = (ord(c) for c in str2)
ord3 = (ord(c) for c in str3)

diffs = (x - y for x,y in zip(ord1, ord2))

result = (chr(x + y) for x,y in zip(diffs, ord3))

print(''.join(result))