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

int main(void){
    double x1, y1, x2, y2, distance;
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    distance = sqrt((pow(x2-x1, 2) + pow(y2-y1, 2)));
    printf("%f\n", distance);
    
    return 0;
}
