#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const int MinC = 1, MaxC = 1000000000;

double a, b, x1, x2;

double f (double x) {return a * x + x / b;}

int main (void)
{
  scanf (" %lf %lf %lf %lf", &a, &b, &x1, &x2);
  assert ((int) a == a && MinC <= a && a <= MaxC);
  assert ((int) b == b && MinC <= b && b <= MaxC);
  assert ((int) x1 == x1 && MinC <= x1 && x1 <= MaxC);
  assert ((int) x2 == x2 && MinC <= x2 && x2 <= MaxC);
  assert (abs (x1 - x2) == 1);
  printf ("%.5lf\n", f (x1) - f (x2));
  return 0;
}

1 2 3 4

-1.5

2 1 4 3

3