/**
 *      Author:  Nicholas Winschel
 *      Created: 11.11.2024 21:39:03
**/

#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using db=long double;
template<class T> using V=vector<T>;
using vi = V<int>;
using vl = V<ll>;
using pi = pair<int,int>;
#define f first
#define s second
#define sz(x) (int)((x).size())
#define each(a,b) for (auto &a : b)

int main() {
  cin.tie(0)->sync_with_stdio(0); // initialize fast I/O

  int n; db t,r; cin >> n >> t >> r;
  db p; cin >> p;
  vector<db> ans(n+1, 1e18), e(n+1, 1e18);
  e[0] = 0;
  for (int i = 1; i <= n; i++) {
    e[i] = 1 + e[i-1] + (e[i-1] + r+1)*(p)/(1-p);
    // cout << "dbg " << e[i] << "\n";
    if (e[i] > 1e18) break;
  } 
  ans[0] = 0;
  for (int c = 1; c <= n; c++) {
    for (int b = 0; b < c; b++) {
      db exp = e[c-b];
      ans[c] = min(ans[b]+t+exp, ans[c]);
    }
  }

  cout << fixed << setprecision(15) << ans[n];

  return 0;
}