#include <stdio.h>

int main() {
  float a11, a12, a21, a22, r, m, n;
  scanf("%f%f%f%f", &a11, &a12, &a21, &a22); /*ucitavamo brojeve*/
  r = a11 * a22;                             /*racunamo determinantu*/
  m = a12 * a21;
  n = r - m;
  printf("%.4f\n", n); /*stampamo*/

}
