n = int(input())
a = list(map(int, input().split()))
a.append(2)
b = []
c0 = c1 = 0
for x in a:
    if x == 0:
        c0 += 1
    elif x == 1:
        c1 += 1
    else:
        if c0 or c1:
            b.append((c0, c1))
            c0 = c1 = 0
INF = 10 ** 9
dp = [INF] * (4 * n + 10)
B = 2 * n + 5
dp[B] = 0
for c0, c1 in b:
    ndp = [INF] * (4 * n + 10)
    for i in range(-n, n):
        if dp[B + i] == INF:
            continue
        ndp[B + i + c1] = min(ndp[B + i + c1], dp[B + i] + c1)
        ndp[B + i - c0] = min(ndp[B + i - c0], dp[B + i] + c0)
    dp = ndp
ans = ndp[B]
if ans == INF:
    ans = -1
else:
    ans //= 2
print(ans)