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

int main(void) {
	// your code goes here
    
    double x1,y1;
    double x2,y2;
    double d;
    
    scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
    d = pow((x1-x2),2)+pow((y1-y2),2);
    d = sqrt(d);
    
    printf("%f",d);
    
	return 0;
}