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 python3
"""
CodeForces
478 A. Initial Bet

@author yamaton
@date 2015-07-29
"""
import sys


def solve(xs):
    total = sum(xs)
    if total % 5 > 0 or total == 0:
        return -1
    else:
        return total // 5


def print_stderr(*args, **kwargs):
    print(*args, file=sys.stderr, **kwargs)


def main():
    cs = [int(i) for i in input().strip().split()]
    assert len(cs) == 5
    result = solve(cs)
    print(result)


if __name__ == '__main__':
    main()
