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

c1 , c2 , c3 , c4 , c5 = input().split()
c1=int(c1)
c2=int(c2)
c3=int(c3)
c4=int(c4)
c5=int(c5)
if(c1>=0 and c1<=100 and c2>=0 and c2<=100 and c3>=0 and c3<=100 and c4>=0 and c4<=100 and c5>=0 and c5<=100):
    total = c1 +c2+ c3 + c4 +c5
    if(total%5==0 and not total==0):
        initial=total/5
        initial=int(initial)
        if(initial!=0):
            print(initial)
        else:
            print("-1")
    else:
        print("-1")