n = int(input())
a = list(map(int, input().split()))
p = [0]
q = [0]
for i in range(n):
    if a[i] == 2:
        p.append(0)
        q.append(0)
    elif a[i] == 1:
        p[-1] += 1
        q[-1] += 1
    else:
        q[-1] += 1
m = len(p)
k = sum(p)
dp = [[n * 2] * (k + 1) for _ in range(m + 1)]
dp[0][0] = 0
for i in range(m):
    for j in range(k + 1):
        if j + q[i] <= k:
            if dp[i + 1][j + q[i]] > dp[i][j]:
                dp[i + 1][j + q[i]] = dp[i][j]
        if dp[i + 1][j] > dp[i][j] + p[i]:
            dp[i + 1][j] = dp[i][j] + p[i]
if dp[m][k] == n * 2:
    print(-1)
else:
    print(dp[m][k])