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


x = map(int,raw_input().split())
w=0
a = sum(x)/5
if sum(x)%5==0 and a!=0:
	
	for i in x:
		w = w +(i-a)
	if w==0:
		print a
	else:
		print -1
else:
	print -1
# # for i in range(input()):

# s  =n-m
# number=0
# maxi = s*(s+1)
# maxi=maxi/2
# j = n/m
# i = n%m
# j = m*(j*(j-1))/2
# j=j+(n/m)*(n%m)
# print j,maxi

