import java.util.*;
public class ShortestPath{
    public static double calculateDistance(int x1,int y1,int x2,int y2){
        return Math.sqrt(Math.abs(Math.pow(x2-x1,2))+ Math.abs(Math.pow(y2-y1,2)));
    }
    public static boolean isValid(int x,int y){
        return(x>= -50 && x<=50 && y>= -50 && y<=50); 
    }
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int x1=sc.nextInt();
        int x2=sc.nextInt();
        int y1=sc.nextInt();
        int y2=sc.nextInt();
        
        if(!isValid(x1,y1) || !isValid(x2,y2)){
            System.out.println("Invalid input");
        }else{
            double distance=calculateDistance(x1,y1,x2,y2);
            if(x==10)
            System.out.printf("%.2f\n",+2*distance);
            else
            System.out.printf("%.2f\n",+distance);
        }
    }
}