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

int main()
{
        double x1=0.0, y1=0.0, x2=0.0, y2=0.0;

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

        return 0;
}