#Fahmi_Nafi_46

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

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