#include <bits/stdc++.h>
long double solve(int n, long double a, long double d) {
  if (!n) return 0;
  long double res = a + d * (2 * n - 1) / 2;
  long double a1 = (a * (n - 1) * 2 + (a + 2 * d) + (3 * a + 3 * d)) / (2 * n);
  long double a2 =
      ((a + d) * (2 * (n - 1) - 1) + (a + 3 * d) * 2 + (3 * a + 6 * d)) /
      (2 * n);
  return res + solve(n - 1, a1, a2 - a1);
}
int main() {
  int n;
  double a, d;
  scanf("%d%lf%lf", &n, &a, &d);
  printf("%.20lf\n", (double)solve(n, a, d));
}
