#include <stdio.h>
#include <math.h>
int main(void)
{
    double x1,y1,x2,y2;
    double x,y;
    scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);
    x = x2 - x1;
    y = y2 - y1;
    
    printf("%lf\n",sqrt(x*x+y*y));
    return 0;
}

