import sys
input = sys.stdin.readline

N=int(input())
A=list(map(int,input().split()))

X=[[0,0]]

for a in A:
    if a==0:
        X[-1][0]+=1
    elif a==1:
        X[-1][1]+=1
    else:
        X.append([0,0])

Y=[]
zeros=0
for x,y in X:
    if x==0 and y==0:
        continue
    zeros+=x
    Y.append([x,y])

DP=[1<<32]*5001
DP[0]=0

for x,y in Y:
    NDP=[1<<32]*5001
    
    for i in range(5000,-1,-1):
        if DP[i]==1<<32:
            continue
        NDP[i+x+y]=min(NDP[i+x+y],DP[i]+y)
        NDP[i]=min(NDP[i],DP[i]+x)

    DP=NDP

if DP[zeros]==1<<32:
    print(-1)
else:
    print(DP[zeros]//2)