mod = 1000000007
eps = 10**-9
inf = 10**15


def main():
    import sys
    input = sys.stdin.readline

    N = int(input())
    A = list(map(int, input().split()))
    A.append(2)

    B = []
    cnt = [0, 0]
    for a in A:
        if a == 2:
            if sum(cnt):
                B.append(cnt)
                cnt = [0, 0]
        else:
            cnt[a] += 1

    S = 0
    for a, b in B:
        S += b

    dp = [inf] * (S+1)
    dp[S] = 0
    for i in range(len(B)):
        n0, n1 = B[i]
        dp_new = [inf] * (S+1)
        for j in range(S+1):
            dp_new[j] = min(dp_new[j], dp[j])
            if j - (n0 + n1) >= 0:
                dp_new[j - (n0 + n1)] = min(dp_new[j - (n0 + n1)], dp[j] + n0)
        dp = dp_new
    if dp[0] != inf:
        print(dp[0])
    else:
        print(-1)


if __name__ == '__main__':
    main()