N = int(input())
A = list(map(int, input().split()))
X, Y = [], []
cntX, cntY = 0, 0
for a in A:
    if a == 2:
        X.append(cntX)
        Y.append(cntY)
        cntX, cntY = 0, 0
    elif a == 0:
        cntX += 1
    else:
        cntY += 1
X.append(cntX)
Y.append(cntY)
M = len(X)
geta = 5005
inf = 10 ** 18
pre = [inf] * (2 * geta)
pre[geta] = 0
for i in range(M):
    dp = [inf] * (2 * geta)
    for j in range(2 * geta):
        if j + X[i] < 2 * geta:
            dp[j + X[i]] = min(dp[j + X[i]], pre[j] + X[i])
        if j - Y[i] >= 0:
            dp[j - Y[i]] = min(dp[j - Y[i]], pre[j])
    dp, pre = pre, dp

print(pre[geta]) if pre[geta] < inf else print(-1)