import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))

def runlength(s):
    if not s:
        return []
    c = s[0]
    v = 1
    n = len(s)
    l = []
    for i in range(1, n):
        if c==s[i]:
            v += 1
        else:
            l.append((c,v))
            c = s[i]
            v = 1
    l.append((c,v))
    return l

n = int(input())
a = list(map(int, input().split()))
res = runlength(a)
l = []
tmp = [0,0]
for c,v in res:
    if c==2:
        if sum(tmp)>0:
            l.append(tmp)
        tmp = [0,0]
    else:
        tmp[c] += v
if sum(tmp)>0:
    l.append(tmp)
d = {0: 0}
for a,b in l:
    nd = {}
    for k,v in d.items():
        nk = k+a
        nv = v
        nd.setdefault(nk,10**12)    
        nd[nk] = min(nd[nk], nv)
        nk = k-b
        nv = v+b
        nd.setdefault(nk,10**12)
        nd[nk] = min(nd[nk], nv)
    d = nd
if 0 in d:
    ans = d[0]
else:
    ans = -1
print(ans)