N = int(input())
A =list(map(int,input().split()))
l = []
zero,one = 0,0

for a,i in zip(A,range(N)):
  if a == 0:
    zero += 1
  elif a == 1:
    one += 1
  else:
    if zero != 0 and one != 0:
      l.append(abs(zero-one))
      zero,one = 0,0
if zero != 0 and one != 0:
  l.append(abs(zero-one))

if l == []:
  print(0)
elif len(l) == 1:
  print(-1)
else:
  l.sort()
  ans = sum(l[:len(l)//2])
  print(ans)