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

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

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

  printf("%f\n",sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2)));
  
  return 0;
}