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

def inp(s):
    j = 0
    a = []
    for i in range(len(s)):
        if s[i] == " ":
            a.append(int(s[j:i]))
            j = i+1
        if i == len(s)-1:
            a.append(int(s[j:]))
    return a

s = input()
a = inp(s)
r = 0
for i in a:
    r = r + i

if r == 0:
    print (-1)

elif r%5 == 0:
    print (int(r/5))
else:
    print (-1)