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

#   FILE: initialBet.py
#   CodeForces 478A

def main():
    a, b, c, d, e, = input().split()
    a, b, c, d, e, = int(a), int(b), int(c), int(d), int(e)

    sum = a + b + c + d + e
    if(sum == 0):
        print(0)
        return()

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

    return()

main()

