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

int main(int argc, const char * argv[]) {
    
    double x1=0.0, x2=0.0;
    double y1=0.0, y2=0.0;
    double dist=0.0;
    
    scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
    
    dist = sqrt(pow(x1-x2, 2.0)+pow(y1-y2, 2.0));
    
    printf("%f\n", dist);
    
    return 0;
}
