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

from sys import stdin, stdout, stderr, setrecursionlimit
setrecursionlimit(100000)

def debug (*e):
    if not __debug__:
        print(*e, file=stderr)

def dd(*vals):
    import inspect, re
    frame = inspect.getframeinfo(inspect.stack()[1][0])
    vs = re.search("dd\((.+)\)", frame.code_context[0]).group(1).split(",")
    if vs:
        debug(",".join("{0} = {1}".format(vs[i], v) for i,v in enumerate(vals)))

def trace(f):
    def traced(*args, **kw):
        debug("calling {} with args {}, {}".format(f.__name__, args, kw))
        return f(*args, **kw)
    return traced
#@trace
#def f(x):

def read():
    return stdin.readline().rstrip()

def readarr(sep=None, maxsplit=-1):
    return read().split(sep, maxsplit)

def readint():
    return int(read())

def readintarr(sep=None, maxsplit=-1):
    return [int(a) for a in readarr(sep, maxsplit)]

#write(1, 2, 3, sep="-", end="!")
def write(*args, **kwargs):
    sep = kwargs.get('sep', ' ')
    end = kwargs.get('end', '\n')
    stdout.write(sep.join(str(a) for a in args) + end)

#writearr([1, 2, 3], sep="-", end="!")
def writearr(arr, sep=' ', end='\n'):
    stdout.write(sep.join(str(a) for a in arr) + end)

import statistics

cs = readintarr()
m = statistics.mean(cs)
if int(m)==m:
    write(int(m))
else:
    write(-1)
