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 __future__ import division
from collections import Counter as ctr
import math
def rl():
	return [int(i) for i in raw_input().split()]

def rm(n=None):
	if n is None:
		n = input()
	return [raw_input() for i in range(n)]

def rlm(n=None):
	if n is None:
		n = input()
	return [rl() for i in range(n)]

def yn(b):
	if b:
		print "YES"
	else:
		print "NO"

l = rl()
if sum(l) % len(l) == 0 and sum(l) != 0:
	print int(sum(l)/len(l))
else:
	print -1