print((min(set(map(chr,list(range(97,123))))-set(eval(input()))or['None'])))
print((min(set(map(chr,list(range(97,123))))-set(eval(input()))or[None])))
print((min(set(map(chr, list(range(97, 123)))) - set(eval(input())) or ["None"])))

print((min(set(map(chr, list(range(97, 123)))) - set(eval(input())) or [None])))

-print((min(set(map(chr, list(range(97, 123)))) - set(eval(input())) or ["None"]))) +print((min(set(map(chr, list(range(97, 123)))) - set(eval(input())) or [None])))