import fileinput
import itertools

N = 25

numbers = []
for line in fileinput.input():
    numbers.append(int(line))

for i in range(N, len(numbers)):
    found = False
    for j in numbers[i - N:i]:
        for k in numbers[i - N:i]:
            if j == k:
                continue
            if j + k == numbers[i]:
                found = True
    if not found:
        print(numbers[i])
        break