DeviaVir

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
        self.prev = None


class LinkedList:
    def __init__(self):
        self.root = None

DeviaVir

def fib():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

        
def main(i=0):
    if i is 0:
        return 0

DeviaVir

def main():
    values = map(int, raw_input().split(',')) 
    return reduce(lambda x, y: x ^ y, values)


print main()

# Because ^ (XOR) returns 0 for each pair and there is only one unpaired item in the input.
# E.g.
# 1 ^ 1 = 0