#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string fld;
  cin >> fld;
  int l = 1, r = 2 * n;
  int ans = -1;
  while (l <= r) {
    int mid = (l + r) >> 1;
    int posbl = 1;
    int back = -1, flag = 0;
    for (int i = 0; i < n; i++) {
      if (fld[i] == '*')
        if (i > back && flag == 0) back = i, flag = 1;
      if (fld[i] == 'P') {
        if (back == -1) {
          back = i + mid;
        } else {
          if (i <= back) {
            back = max(back, i + mid);
          } else {
            if (flag == 1) {
              if (i - back > mid) {
                posbl = 0;
                break;
              } else if (i - back <= mid && mid <= (2 * (i - back)))
                back = max(i, (mid - (i - back)) / 2 + i);
              else {
                back =
                    max(mid - (2 * (i - back)) + i, (mid - (i - back)) / 2 + i);
              }
            } else {
              back = i + mid;
            }
          }
        }
        flag = 0;
      }
    }
    if (flag == 1) posbl = 0;
    if (posbl == 1) {
      r = mid - 1;
      ans = mid;
    } else
      l = mid + 1;
  }
  cout << ans << "\n";
  return 0;
}
