n = int(input())
A = list(map(int, input().split()))
A += [2]
s = 0
for a in A:
    s += (a == 0)
C = []
cnt0 = 0
cnt1 = 0
for i, a in enumerate(A):
    if a == 2:
        if cnt0 or cnt1:
            C.append((cnt0, cnt1))
            cnt0 = 0
            cnt1 = 0
    elif a == 0:
        cnt0 += 1
    else:
        cnt1 += 1

INF = 10**18
dp = [INF]*(s+1)
dp[0] = 0
for c0, c1 in C:
    for j in reversed(range(s+1)):
        if j+c0+c1 <= s:
            dp[j+c0+c1] = min(dp[j+c0+c1], dp[j]+c1)
if dp[s] < INF:
    print(dp[s])
else:
    print(-1)