#include <bits/stdc++.h>
using namespace std;
int n, k, r;
double t, v, turn, ang, bet, ans, pi = 3.14159265358979323846264338;
int main()
{
    cin >> n >> k >> t >> v; 
    turn = t * v * 1.0000000;
    ang = turn * pi;
    bet = sin(ang);
    if(bet < 0.00000)bet = -1.000000 * bet;
    r = abs(n - k);
    ans = 2.000000 * bet * double(r);
    cout << fixed << setprecision(10) << ans;
    return 0;
}