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

int main(int argc, char *argv[])
{
  double x1, y1, x2, y2;
  double dis;

  scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

  dis = sqrt((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1));

  printf("%.8lf\n", dis);
  
  return 0;
}