from sys import stdin
n, *a = map(int, stdin.read().split())
niku = []
nonbird = []
nowniku = 0
nownonbird = 0
sumniku = 0
for i in range(n):
    if a[i] == 0:
        nowniku += 1
        sumniku += 1
        nownonbird += 1
    elif a[i] == 1:
        nownonbird += 1
    else:
        if nownonbird != 0:
            niku.append(nowniku)
            nonbird.append(nownonbird)
            nowniku = 0
            nownonbird = 0

if nownonbird != 0:
    niku.append(nowniku)
    nonbird.append(nownonbird)

m = len(niku)
dp = [[n+1 for i in range(sumniku+1)] for j in range(m+1)]
dp[0][0] = 0
for i in range(m):
    for j in range(sumniku+1):
        dp[i+1][j] = min(dp[i+1][j],dp[i][j] + niku[i])
        if j >= nonbird[i]:
            dp[i+1][j] = min(dp[i+1][j],dp[i][j-nonbird[i]] + nonbird[i] - niku[i])

if dp[m][sumniku] == n+1:
    print("{}".format(-1))
else:
    print("{}".format(dp[m][sumniku]//2))