#Nafi_62_i

n = int(input())
lst = list(map(int,input().split()))
odd = []
even = []
odd_len = 0
for i in lst:
    if i % 2 == 0:
        if odd_len == 0:
            even.append(i)
    else:
        odd.append(i)
        odd_len += 1
        jug = sum(odd)
        if jug % 2 == 0:
            odd_len = 0
            even.append(jug)
        else:
            continue
if odd_len == 0:
    ans = len(even)
else:
    ans = -1
print(ans)