n = int(input())
alst = list(map(int, input().split()))
one = 0
vw = []
tot = 0
cnt = 0
for a in alst:
    if a == 2:
        if tot != 0:
            vw.append((cnt, tot))
        cnt = tot = 0
    elif a == 0:
        tot += 1
    else:
        cnt += 1
        tot += 1
        one += 1
if tot != 0:
    vw.append((cnt, tot))
dp = [-10 ** 20 for _ in range(one + 1)]
dp[0] = 0
for v, w in vw:
    for j in range(one, w - 1, -1):
        dp[j] = max(dp[j], dp[j - w] + v)

if dp[one] < 0:
    print(-1)
else:
    print(one - dp[-1])