99 100 100 100 100
 57 83 11 4 93
 99 99 99 99 99
 100 0 0 0 0
 0 1 2 3 4
 93 100 99 90 98
 87 38 19 33 100
 1 1 1 1 1
 0 0 0 0 1
 2 3 4 5 6
 1 2 1 2 3
 0 0 0 0 0
 100 100 100 100 100
 43 83 1 0 23
 43 83 1 100 23
 56 0 0 0 4
 99 98 98 99 100

-1
 -1
 99
 20
 2
 96
 -1
 1
 -1
 4
 -1
 -1
 100
 30
 50
 12
 -1

__author__ = 'Devesh Bajpai'

'''
https://codeforces.com/problemset/problem/478/A

Solution: Since every player is given same amount of coins, the total coins at the end of the game should still be 
divisible by the total number of players. Check and return the result accordingly. 
'''

def solve (num):

    n = len(num)
    sum = 0
    for num in nums:
        sum += num

    return -1 if sum == 0 or sum % n != 0 else sum / n



if __name__ == "__main__":

    nums = map(int,raw_input().split(" "))
    print solve(nums)
