//#define NDEBUG
#pragma warning(disable : 4146)

#include <algorithm>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <iomanip>
#include <ios>
#include <iostream>
#include <utility>
#include <vector>

namespace n91 {

  using i32 = std::int32_t;
  using i64 = std::int64_t;
  using u32 = std::uint32_t;
  using u64 = std::uint64_t;
  using isize = std::ptrdiff_t;
  using usize = std::size_t;
  using f64 = double;

  struct rep {
    struct itr {
      usize i;
      constexpr itr(const usize i) noexcept : i(i) {}
      void operator++() noexcept { ++i; }
      constexpr usize operator*() const noexcept { return i; }
      constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
    };
    const itr f, l;
    constexpr rep(const usize f, const usize l) noexcept
      : f(std::min(f, l)), l(l) {}
    constexpr auto begin() const noexcept { return f; }
    constexpr auto end() const noexcept { return l; }
  };
  struct revrep {
    struct itr {
      usize i;
      constexpr itr(const usize i) noexcept : i(i) {}
      void operator++() noexcept { --i; }
      constexpr usize operator*() const noexcept { return i; }
      constexpr bool operator!=(const itr x) const noexcept { return i != x.i; }
    };
    const itr f, l;
    constexpr revrep(const usize f, const usize l) noexcept
      : f(l - 1), l(std::min(f, l) - 1) {}
    constexpr auto begin() const noexcept { return f; }
    constexpr auto end() const noexcept { return l; }
  };
  template <class T> auto md_vec(const usize n, const T& value) {
    return std::vector<T>(n, value);
  }
  template <class... Args> auto md_vec(const usize n, Args... args) {
    return std::vector<decltype(md_vec(args...))>(n, md_vec(args...));
  }
  template <class T> constexpr T difference(const T& a, const T& b) noexcept {
    return a < b ? b - a : a - b;
  }
  template <class T> void chmin(T& a, const T& b) noexcept {
    if (b < a)
      a = b;
  }
  template <class T> void chmax(T& a, const T& b) noexcept {
    if (a < b)
      a = b;
  }
  template <class F> class rec_lambda {
    F f;

  public:
    rec_lambda(F&& f_) : f(std::forward<F>(f_)) {}
    template <class... Args> auto operator()(Args &&... args) const {
      return f(*this, std::forward<Args>(args)...);
    }
  };
  template <class T> T scan() {
    T ret;
    std::cin >> ret;
    return ret;
  }
  constexpr char eoln = '\n';

  i64 floor_div(const i64 n, const i64 d) {
    assert(d != 0);
    return n / d - static_cast<i64>((n ^ d) < 0 && n % d != 0);
  }

  i64 ceil_div(const i64 n, const i64 d) {
    assert(d != 0);
    return n / d + static_cast<i64>((n ^ d) >= 0 && n % d != 0);
  }

#ifdef N91_LOCAL
#define OJ_LOCAL(a, b) b
#else
#define OJ_LOCAL(a, b) a
#endif

} // namespace n91

namespace n91 {

  void main_() {
    const usize n = scan<usize>();
    const std::string s = scan<std::string>() + '#';
    std::vector<f64> f(n + 1, 0), g0(n + 1, 0), g1(n + 1, 0);
    for (const usize i : revrep(0, n + 1)) {
      if (i == n - 1) {
        continue;
      }
      if (s[i] == '#') {
          g1[i] = 1;
          continue;
      }
      f[i] = (g0[i + 1] + g0[i + 2] + 3) / (2 - g1[i + 1] - g1[i + 2]);
      g0[i] = (g0[i + 1] + g0[i + 2] + f[i]) / 3 + 1;
      g1[i] = (g1[i + 1] + g1[i + 2]) / 3;
    }

    std::cout << f[0] << eoln;
  }

} // namespace n91

int main() {
  //*
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  //*/
  std::cout << std::fixed << std::setprecision(20);
  n91::main_();
  return 0;
}