def oi(): return int(input())
def os(): return input()
def mi(): return list(map(int, input().split()))

# import sys
# input = sys.stdin.readline
# import sys
# sys.setrecursionlimit(10**8)
# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')

input_count = 0
N = oi()
A = mi()
elements = []
temp = {0:0, 1:0}
for a in A:
    if a == 2:
        if len(temp)!=0:
            elements.append(temp)
        temp = {0:0, 1:0}
    else:
        temp[a] += 1            
if len(temp)!=0:
    elements.append(temp)
one_sum = A.count(1)
diffs = []
for e in elements:
    diffs.append([e[0] + e[1], e[0]])
# 一致してなければDP
INF = 10**5
dp = [[INF] * (N+2) for _ in range(len(elements)+1)]

dp[0][0] = 0
for y,d in enumerate(diffs):
    for x in range(0, N+2):
        dp[y+1][x] = min(dp[y][x], dp[y+1][x])
        if dp[y][x] != INF:
            if 0 <= x+d[0] <= 2*N+1:
                dp[y+1][x+d[0]] = min(dp[y+1][x+d[0]], dp[y][x]+d[1])
                


ret = dp[-1][one_sum]
if ret != INF:
    print(ret)
else:
    print(-1)