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 gcd(a,b):
    if a == 0: return b
    return gcd(b % a, a)

def solve():
    a,b,c,d,e = map(int, input().split())
    # print(a,b,c,d,e)
    sum = a + b + c + d + e

    if sum == 0: print(-1); return

    if sum % 5 ==0:
        print(int(sum/5))
    else:
        print(-1)

solve()
    # C = [0]; ptr = 0; i = 1; begin = False
# while i < len(B):
#     if B[i] == B[ptr]:
#         ptr += 1
#         C.append(ptr)
#         i += 1
#     else:
#         # Update till prefix matched
#         if ptr != 0:
#             ptr = C[ptr-1]
#             # Also no i increment
#         else:
#             C.append(0)
#             i += 1
# print(C)
