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

#coding: utf-8

moedas = raw_input().split()
coin1 = int(moedas[0])
coin2 = int(moedas[1])
coin3 = int(moedas[2])
coin4 = int(moedas[3])
coin5 = int(moedas[4])

soma = coin1 + coin2 + coin3 + coin4 + coin5
valor = soma / 5

if coin1 != 0 and coin2 != 0 and coin3 != 0 and coin4 != 0 and coin5 != 0:
   print "-1"
elif soma % 5 == 0:
   print "%d" % valor
elif soma % 5 != 0:
      print "-1"



