N = int(input())
A = list(map(int,input().split()))
A.insert(0,2)
A.append(2)
N += 2
l = []
left = 0
S = 0
while left < N:
    right = left + 1
    count = 0
    while right < N and A[right] != 2:
        if A[right] == 0:
            count += 1
        right += 1
    if right - left > 1:
        l.append((right- left - 1,right - left - 1 - count))
    S += count
    left = right
n = len(l)
inf = 10 ** 6
dp = [[inf] * (S+1) for _ in range(n+1)]
dp[0][0] = 0
for i in range(n):
    w,v = l[i]
    for j in range(S+1):
        dp[i+1][j] = min(dp[i+1][j],dp[i][j])
        if j + w <= S:
            dp[i+1][j+w] = min(dp[i+1][j+w],dp[i][j] + v)
if dp[-1][-1] == inf:
    print(-1)
else:
    print(dp[-1][-1])