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

#!/usr/bin/env python

def main ():
    arr = map(lambda t: int(t), raw_input().split())
    n = len(arr)
    s = sum(arr)
    allzero = all(t == 0 for t in arr)

    if allzero:
        print -1
    elif s % n != 0:
        print -1
    else:
        print s/n

if __name__ == "__main__":
    main ()

