from sys import exit, stdin, setrecursionlimit
from collections import deque, defaultdict, Counter
from copy import deepcopy
from bisect import bisect_left, bisect_right, insort_left, insort_right
from heapq import heapify, heappop, heappush
from itertools import product, permutations, combinations, combinations_with_replacement
from functools import reduce
from math import gcd, sin, cos, tan, asin, acos, atan, atan2, degrees, radians, ceil, floor, sqrt, factorial
from math import pi as PI
from random import randint
# from decimal import Decimal

setrecursionlimit(500000)
INF = (1<<61)-1
EPS = 1e-10
MOD = 10**9+7
# MOD = 998244353

def input():
    return stdin.readline()[:-1]
def intput():
    return int(stdin.readline())
def minput():
    return stdin.readline()[:-1].split()
def linput():
    return stdin.readline()[:-1].split()
def mint():
    return map(int,stdin.readline().split())
def lint():
    return list(map(int,stdin.readline().split()))
def ilint():
    return int(stdin.readline()), list(map(int,stdin.readline().split()))
def lcm(x,y):
    return x*y//gcd(x,y)
def lgcd(l):
    return reduce(gcd,l)
def llcm(l):
    return reduce(lcm,l)
def powmod(n,i,mod=MOD):
    return pow(n,mod-1+i,mod) if i<0 else pow(n,i,mod)
def div2(x):
    return x.bit_length()
def div10(x):
    return len(str(x))-(x==0)
def popcount(x):
    return bin(x).count('1')
def digit(x,i,max_len=None):
    s = str(x)
    if max_len:
        i -= max_len-len(s)
    return int(s[i-1]) if i>0 else 0
def digitsum(x):
    ans = 0
    for i in range(div10(x)):
        ans += digit(x,i+1)
    return ans
def pf(x,mode='counter'):
    C = Counter()
    p = 2
    while x>1:
        k = 0
        while x%p==0:
            x //= p
            k += 1
        if k>0:
            C[p] += k
        p = p+2-(p==2) if p*p<x else x
    if mode=='counter':
        return C
    S = set([1])
    for k in C:
        T = set()
        for x in S:
            for i in range(C[k]+1):
                T.add(x*(k**i))
        S = T
    if mode=='set':
        return S
    if mode=='list':
        return sorted(S)
def isprime(x):
    if x<2:
        return False
    return len(pf(x,'set'))==2
def matmul(A, B):
    # import numpy
    A1, A2 = A >> 15, A & (1 << 15) - 1
    B1, B2 = B >> 15, B & (1 << 15) - 1
    X = np.dot(A1, B1) % MOD
    Y = np.dot(A2, B2)
    Z = np.dot(A1 + A2, B1 + B2) - X - Y
    return ((X << 30) + (Z << 15) + Y) % MOD
def matpow(A, N):
    P = np.eye(A.shape[0], dtype=np.int64)
    while N:
        if N & 1:
            P = matmul(P, A)
        A = matmul(A, A)
        N >>= 1
    return P
def zash(S):
    lis = sorted(S)
    dic = {}
    for i,x in enumerate(lis):
        dic[x] = i
    return lis, dic
def pr(*x):
    print(*x, sep='', end='') if len(x) else print()
def lprint(l):
    for x in l: print(x)
def ston(c, c0='a'):
    return ord(c)-ord(c0)
def ntos(x, c0='a'):
    return chr(x+ord(c0))
def judge(x, l=['Yes', 'No']):
    print(l[0] if x else l[1])
def debug(*x, flag=1):
    if flag: print(*x)

######################################################

N,A=ilint()
M=5000
l=[]
left=0
right=0
all_right=0
for a in A:
    if a==0:
        left+=1
        right+=1
    elif a==1:
        left+=1
    elif left>0:
        l.append([left,right])
        all_right+=right
        left=0
        right=0
if left>0:
    l.append([left,right])
    all_right+=right
dp=[INF]*(M+1)
dp[0]=0
for left,right in l:
    nxt=[INF]*(M+1)
    for v in range(M+1):
        if dp[v]<INF:
            nxt[v]=min(nxt[v],dp[v])
            nxt[v+left]=min(nxt[v+left],dp[v]+left-right)
    dp=nxt
ans=dp[all_right]
print(ans if ans<INF else -1)