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

