import sys

sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 10**16
md = 998244353
# md = 10**9+7

n = II()
aa = LI()+[2]
c0 = c1 = 0
s = 0
seg = []

for i, a in enumerate(aa):
    if a == 0:
        c0 += 1
        s += 1
    elif a == 1:
        c1 += 1
    elif c0+c1:
        seg.append((c0, c1))
        c0 = c1 = 0
# print(seg)

def chmin(j, val):
    if val < dp[j]: dp[j] = val

m = len(seg)
dp = [inf]*(s+1)
dp[0] = 0
for i, (c0, c1) in enumerate(seg):
    for j in range(s-c0-c1+1)[::-1]:
        if dp[j] == inf: continue
        chmin(j+c0+c1, dp[j]+c1)
    # print(dp)

ans = dp[-1]
if ans == inf: ans = -1
print(ans)