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

"""
There are five people playing a game called "Generosity". Each person gives some
non-zero number of coins b as an initial bet. After all players make their bets
of b coins, the following operation is repeated for several times: a coin is passed
from one player to some other player.

Your task is to write a program that can, given the number of coins each player
has at the end of the game, determine the size b of the initial bet or find out
that such outcome of the game cannot be obtained for any positive number of coins
b in the initial bet.
"""

remcoins = map(int, raw_input().split())
ans = sum(remcoins) / 5
if ans * 5 != sum(remcoins) or ans == 0: print -1
else: print ans