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

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

    scanf( "%lf%lf%lf%lf", &x1, &y1, &x2, &y2 );
    x1=(x1-x2);
    x1 = (x1 < 0)? -1*(x1): x1;
    y1=(y1-y2);   
    y1 = (y1 < 0)? -1*(y1): y1;
    distance = sqrt( pow( x1, 2 )+pow( y1, 2 ) );
	printf( "%.8lf\n", distance );

	return 0;
}