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