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 itertools import product

given = (int(x) for x in input().split(' '))

#given = tuple(given)
#c1,c2,c3,c4,c5 = given

def main():
	s= sum(given)
	
	if s<5:
		print(-1)
		return

	if s%5 == 0:
		print(s // 5)
	else:
		print(-1)

main()

#glb = None
#def fprint(a):
	#global glb
	#glb = a
	

#r = range(0, 11)

#i = 0
#for x in product(r,r,r,r,r):
	#given = x
	#main()
	#if glb >= 10:
		#print(glb, x)
	
	#i+=1
	##if i > 50: break
	##print(glb, x)