N=int(input())
A=list(map(int, input().split()))
A0,A1=[0],[0]
for a in A:
    if a==0:
        A0[-1]+=1
    elif a==1:
        A1[-1]+=1
    else:
        A0.append(0)
        A1.append(0)
C0=sum(A0)
INF=10**9
dp=[INF]*(C0+1)
dp[0]=0
for a0,a1 in zip(A0,A1):
    for c in range(C0+1)[::-1]:
        if c-(a0+a1)>=0 and dp[c-(a0+a1)]<INF:
            dp[c]=min(dp[c-(a0+a1)]+a1,dp[c]+a0)
        elif dp[c]<INF:
            dp[c]+=a0


print(dp[-1]//2 if dp[-1]<INF else -1)