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

