n = int(input()) *a, = map(int,input().split()) a.append(2) res = [] c = d = 0 for ai in a: if ai == 0: c += 1 elif ai == 1: d += 1 else: if c or d: res.append((c,d)) c = d = 0 dp = {0:0} # 差が k になる最小回数が v from collections import defaultdict for c,d in res: ndp = defaultdict(lambda :100000) for k,v in dp.items(): ndp[k+c] = min(ndp[k+c],v+c) ndp[k-d] = min(ndp[k-d],v+d) dp = ndp if 0 in dp: print(dp[0]//2) else: print(-1)