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

int main(void){

  double xa, ya, xb, yb;
  double x = 0;
  double y = 0;
  double xxyy= 0;
  double xy = 0;
  
  scanf("%lf%lf%lf%lf", &xa, &ya, &xb, &yb);

  x = xa - xb;
  y = ya - yb;
  xxyy = x*x + y*y;
  xy = sqrt(xxyy);

  printf("%f\n", xy);
  return 0;
}