n=int(input())
a=list(map(int, input().split()))
b=[]
L=0
O=0
alllen=0
allone=0
for i in range(n):
    if a[i] == 2:
        if L>0:
            if 0<O<L:
                b.append([L, O])
            L=0
            O=0
    else:
        alllen+=1
        L+=1
        if a[i]==1:
            allone+=1
            O+=1
if L>0:
    b.append([L, O])

m=len(b)
o=n*2+10
dp=[[1e9]*(o) for _ in range(m+1)]
dp[0][n]=0
for i in range(m):
    len, one = b[i]
    zero = len - one
    for j in range(o):
        if dp[i][j]==1e9:
            continue
        if j + one < o:
            dp[i+1][j+one] = min(dp[i+1][j+one], dp[i][j] + one)
        if j - zero >= 0:
            dp[i+1][j-zero]=min(dp[i+1][j-zero],dp[i][j])

if dp[m][n] == 1e9:
    print(-1)
else:
    print(dp[m][n])