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

temp = raw_input().split(" ")
# #
nums = [ int( t ) for t in temp ]
# #
sm = sum( nums )
if sm == 0:
    print - 1
else:

    if sm % 5 == 0:
        print sm / 5
    else:
        print -1


# temp = raw_input().split(" ")
#
# n = int( temp[ 0 ] ) # players
# m = int( temp[ 1 ] ) # teams
#
#
# min_per_team = n / m
# add = n % m
#
#
# if add == 0:
#     mn = m * ( min_per_team * ( min_per_team - 1 ) ) / 2
# else:
#     mn = ( m - add ) * ( ( min_per_team * ( min_per_team - 1 ) ) / 2 )
#     min_per_team += 1
#     mn +=  add * ( ( min_per_team * ( min_per_team - 1 ) ) / 2 )
#
# print mn,
#
#
# max_per_one_team = n - ( m - 1 )
#
# mx = ( max_per_one_team * ( max_per_one_team - 1 ) ) / 2
#
#
# print mx

# temp = raw_input().split(" ")
#
# r = int( temp[ 0 ] )
# g = int( temp[ 1 ] )
# b = int( temp[ 1 ] )

