N = int(input())
arr = list(map(int, input().split()))
oddCount = 0
even_count = 0
none=0
for i in range(N):
    if arr[i] % 2 != 0:
        oddCount += 1
    elif arr[i]==0:
      none+=1
    else:
        even_count += 1
if oddCount % 2 != 0:
    print(-1)
else:
    print(oddCount // 2 + even_count)