from collections import defaultdict
N = int(input())
A = list(map(int,input().split()))
X = []
tmp = [0,0]
for a in A:
    if a == 2:
        if tmp != [0,0]:
            X.append(tmp)
            tmp = [0,0]
    else:
        tmp[a] += 1
if tmp != [0,0]:
    X.append(tmp)
NX = len(X)
dic1 = {0:0}
for x in X:
    dic2 = defaultdict(lambda:2 * N)
    a,b = x
    for k,v in dic1.items():
        dic2[k-a] = min(dic2[k-a],v + a)
        dic2[k+b] = min(dic2[k+b],v + b)
    dic1 = dic2
if dic1[0] == 2 * N:
    print(-1)
else:
    print(dic1[0]//2)