import collections
N = int(input())
A = list(map(int,input().split()))
ZERO = A.count(0)
ONE = A.count(1)
L = []
SEP = []
for a in A:
    if a==2:
        if len(SEP)>0:
            L.append(SEP)
            SEP = []
    else:
        SEP.append(a)
L.append(SEP)
#コストを設定
DP = collections.defaultdict(lambda:10**4)
DP[(0,0)]=0
for l in L:
    NDP = collections.defaultdict(lambda:10**4)
    CA = collections.Counter(l)
    for k,v in DP.items():
        z,o = k
        if z+len(l)<=ZERO:
            NDP[(z+len(l),o)]=min(NDP[(z+len(l),o)],v+CA[1])
        if o+len(l)<=ONE:
            NDP[(z,o+len(l))]=min(NDP[(z,o+len(l))],v+CA[0])
    DP = NDP
if DP[(ZERO,ONE)]==10**4:
    print(-1)
else:
    print(DP[(ZERO,ONE)]//2)