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

lista = list(map(int,input().split()))
mayor = max(lista)
bolsa = 0
find = False
val = 0
lista.sort()
for i in range(1,mayor+1):
    bolsa = 0
    for j in range(5):
        if i > lista[j]:
            bolsa-=i-lista[j]
        elif i < lista[j]:
            bolsa+=lista[j]-i
    if bolsa == 0:
        val = i
        find = True
        break
    
    if find:
        break
if find:
    print(i)
else:
    print('-1')
        
        