#include <bits/stdc++.h>

using namespace std;

const int Nmax = 2005;

long double e[Nmax], c[Nmax], t, r, p;

int n, i, j;

int main()
{
    //freopen("input", "r", stdin);
    cin.tie(0); cin.sync_with_stdio(false);

    cin >> n >> t >> r >> p;

    for(i=1; i<=n; ++i)
        c[i] =  (c[i-1] + 1 + p * r) / (1-p);

    for(i=1; i<=n; ++i)
    {
        e[i] = c[i] + t;

        for(j=1; j<i; ++j)
            e[i] = min(e[i], e[j] + c[i-j] + t);
    }

    cout << setprecision(6) << fixed;
    cout << e[n];

    return 0;
}