n = int(input())
x = list(map(int, input().split()))
count = 0
odd = 0

for i in range(n):
    if x[i] % 2 != 0:
        odd += 1
if odd % 2 != 0:
    print(-1)
else:
    for i in range(n):
        if x[i] % 2 == 0:
            count += 1
        else:
            odd += 1
            if odd % 2 == 0:
                count += 1
    print(count)