import sys sys.setrecursionlimit(10**6) int1 = lambda x: int(x)-1 p2D = lambda x: print(*x, sep="\n") def II(): return int(sys.stdin.buffer.readline()) def MI(): return map(int, sys.stdin.buffer.readline().split()) def MI1(): return map(int1, sys.stdin.buffer.readline().split()) def LI(): return list(map(int, sys.stdin.buffer.readline().split())) def LI1(): return list(map(int1, sys.stdin.buffer.readline().split())) def LLI(rows_number): return [LI() for _ in range(rows_number)] def BI(): return sys.stdin.buffer.readline().rstrip() def SI(): return sys.stdin.buffer.readline().rstrip().decode() dij = [(0, 1), (-1, 0), (0, -1), (1, 0)] inf = 10**19 # md = 998244353 md = 10**9+7 from collections import Counter s=SI() cc=Counter(s) for k,v in cc.items(): if v==1: print(k) exit()