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

int main(int argc, const char * argv[]) {
    // insert code here...
    double x1,y1,x2,y2;
    scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);

    printf("%f\n",sqrt(pow(x2- x1, 2) +pow(y2 -y1, 2)));
    
    
    return 0;
}