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

from collections import deque, Counter, OrderedDict
from heapq import nsmallest, nlargest
def binNumber(n,size):
    return bin(n)[2:].zfill(size)

def gcd(a,b):
    if a == 0:
        return b
    return gcd(b%a,a)

#  =========     /\       /|    |====/|
#      |        /  \       |    |   / |
#      |       /____\      |    |  /  |
#      |      /      \     |    | /   |
#  ========= /        \  =====  |/====|  
#  code
if __name__ == "__main__":
    a = list(map(int,input().split()))
    if sum(a)%len(a) == 0 and a != [0]*len(a):
        print(sum(a)//len(a))
    else:
        print(-1)