n, k = read_line.split.map(&.to_i)
s = "o" + read_line

dp = [false] * (n + 1)
cnt = 0
(0...n).reverse_each do |i|
  if s[i] == 'o' && cnt == 0
    dp[i] = true
  end
  cnt -= 1 if i + k <= n && dp[i + k]
  cnt += 1 if dp[i]
end
puts (dp[1, k].index(true) || -1) + 1