import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
sys.setrecursionlimit(10**7)
yes = lambda :print("yes");Yes = lambda :print("Yes")
no = lambda :print("no");No = lambda :print("No")
#################################################
n = ni()
a = na()+[2]
A = []
B = []
x,y = 0,0
for i in range(n+1):
    if a[i]==2:
        if x==0 and y==0:continue
        A.append(x)
        B.append(y)
        x = 0
        y = 0
    elif a[i]==0:
        x+=1
    else:
        y+=1
S = sum(B)
N = len(A)
AB = [A[i]+B[i] for i in range(N)]
inf = 10**15
dp = [[inf for j in range(S+1)]for i in range(N+1)]
dp[0][0] = 0
for i in range(N):
    for j in range(S+1):
        if j-AB[i]>=0:
            dp[i+1][j] = min(dp[i+1][j],dp[i][j-AB[i]]+A[i])
        dp[i+1][j] = min(dp[i+1][j],dp[i][j])

if dp[-1][-1]==inf:
    print(-1)
else:
    print(dp[-1][-1])