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

for a in A:
  if a == 0:
    zero += 1
  elif a == 1:
    one += 1
  else:
    if zero != 0 and one != 0:
      l.append([zero,one])
    zero,one = 0,0
if zero != 0 and one != 0:
  l.append([zero,one])

ans = 0
zero,one = 0,0

if len(l) == 1:
  print(-1)
  exit()
for z,o in l:
  if z < zero:
    ans += (zero-z)//2
    zero -= z
  elif o < one:
    ans += (one-o)//2
    one -= o
  else:
    zero += z
    one += o
    if zero == one:
      zero = one = 0

if zero == one:
  print(ans)
else:
  print(-1)