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

import sys

class Scanner:

    def __init__(self):
        self.current_tokens = []

    def remaining_tokens(self):
        return len(self.current_tokens)

    def nextline(self):
        assert self.remaining_tokens() == 0, "Reading next line with remaining tokens"
        return input()

    def nexttokens(self):
        return self.nextline().split()
    
    def nexttoken(self):
        if len(self.current_tokens) == 0:
            self.current_tokens = self.nexttokens()
        assert self.remaining_tokens() > 0, "Not enough tokens to parse."
        return self.current_tokens.pop(0)

    def nextints(self, n=-1):
        if n == -1:
            return list(map(int, self.nexttokens()))
        else:
            return (self.nextint() for i in range(n))
    
    def nextint(self):
        return int(self.nexttoken())

def quit():
    sys.exit(0)

stdin = Scanner()
nextint = stdin.nextint
nextints = stdin.nextints
nextline = stdin.nextline

c = sum(nextints())
if c % 5 == 0 and c > 0:
    print(c // 5)
else:
    print("-1")
