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

int main() {

double x1,x2,y1,y2;
double x,y,z;
scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);

x = (x2-x1) * (x2-x1);
y = (y2-y1) * (y2-y1);

printf("%.11lf",sqrt(x+y));


return 0;

}

