X=input()
Y=input()

if len(X) - len(Y) not in (0,1):
    print("?")
    quit()

S = ''
for i in range(len(X)+len(Y)):
    if i % 2 == 0:
        S += X[i//2]
    else:
        S += Y[(i-1)//2]
print(S)