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

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

    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    
    printf("%lf\n", sqrt(pow((x2-x1),2)+pow((y2-y1),2)));

    return 0;
}