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

int main(void){
  double x1, x2, y1, y2;
  double dis;

  scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
  dis = sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2));
  printf("%lf\n", dis);
  return 0;
}

