#include <stdio.h>
#include <math.h>

int main(void){

  int x1, y1, x2, y2;
  float result;

  scanf("%d %d %d %d", &x1, &y1, &x2, &y2);
  result = sqrt ((x1 - x2)*(x1 - x2) +  (y1 - y2)*(y1 - y2));
  printf ("%f\n", result);

  return 0;

}