#include <bits/stdc++.h>

using namespace std;

const int INF = 1 << 30;

int solve(string S, int N)
{
  int right = -1, ret = 0;
  for(int i = N; i >= 0; i--) {
    if(S[i] == '#') right = i;
  }
  if(right == -1) return (INF);
  for(int i = N - 1; i >= right; i--) {
    S[i] = ".#"[S[i] == '.'];
    ++ret;
  }
  S += ".";
  int rest = count(begin(S), end(S), '#');
  while(rest > 0) {
    ++right;
    assert(right < S.size());
    ++ret;
    S[right] = ".#"[S[right] == '.'];
    if(S[right] == '.') rest--;
    else rest++;
    if(S[right - 1] == '#') {
      --right;
      ++ret;
      S[right] = ".#"[S[right] == '.'];
      if(S[right] == '.') rest--;
      else rest++;
    }
  }
  return (ret);
}

int main()
{
  int N;
  string S, T;
  cin >> N;
  cin >> S;

  if(N > 20) {
    T = S;
    reverse(begin(T), end(T));
    cout << min(solve(S, N - 1), solve(T, (int) S.size() - N)) << endl;
    return (0);
  }

  --N;
  queue< pair< int, string > > que;
  map< string, int > min_cost[S.size()];
  min_cost[N][S] = 0;
  que.emplace(N, S);
  while(!que.empty()) {
    int idx;
    string s;
    tie(idx, s) = que.front();
    que.pop();
    if(count(begin(s), end(s), '#') == 0) {
      cout << min_cost[idx][s] << endl;
      break;
    }
    for(int i = -1; i <= 1; i++) {
      if(i == 0) continue;
      if(idx + i < 0 || idx + i >= S.size()) continue;
      string p = s;
      p[idx + i] = ".#"[p[idx + i] == '.'];
      if(min_cost[idx + i].count(p)) continue;
      min_cost[idx + i][p] = min_cost[idx][s] + 1;
      que.emplace(idx + i, p);
    }
  }

}