n = int(input())
x = list(map(int, input().split()))
count = 0
odd = 0
new = []
for i in range(n):
    if x[i] % 2 != 0:
        odd += 1
if odd % 2 != 0:
    print(-1)
else:
    z = set(x)
    for i in z:
        if i % 2 == 0:
            count += 1
    print(count+(odd//2))