n = int(input()); s = ""
for i in range(n):
    a = []
    for j in range(25):
        print("?","a"*i+chr(j+97)+"a"*(n-i-1))
        a.append(int(input()))
    if min(a)==max(a): s += "z"
    else: s += chr(a.index(max(a))+97)
print(s)