import sys
input = sys.stdin.readline

N = int(input())
A = list(map(int, input().split()))
l = []
niku, sou = 0, 0

for i in range(N):
    if A[i]==0:
        niku += 1
    elif A[i]==1:
        sou += 1
    else:
        l.append((niku, sou))
        niku, sou = 0, 0

l.append((niku, sou))
niku_S = A.count(0)
dp = [[10**18]*(niku_S+1) for _ in range(len(l)+1)]
dp[0][0] = 0

for i in range(len(l)):
    for j in range(niku_S+1):
        dp[i+1][j] = min(dp[i+1][j], dp[i][j]+l[i][0])
        
        if j+sum(l[i])<=niku_S:
            dp[i+1][j+sum(l[i])] = min(dp[i+1][j+sum(l[i])], dp[i][j]+l[i][1])

if dp[-1][-1]==10**18:
    print(-1)
else:
    ans = dp[len(l)][niku_S]//2
    print(ans)