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

l=list(map(int, input().strip().split()))
if sum(l) == max(l):
    print(max(l)//5)
else:
    #print(l)
    l[3] = l[3]+1
    l[1] = l[1]-1
    #second
    l[0] = l[0] +1
    l[2] = l[2] -1
    #third
    l[3] = l[3] +1
    l[4] = l[4] -1
    #fourth
    l[3] = l[3] +1
    l[1] = l[1] -1
    count=1
    #print(l)
    for i in range(len(l)-1):
        if l[i] == l[i+1]:
            count = count+1

    if count == 5:
        print(l[0])     
    else:
        print("-1")

