#include <stdio.h>
double D(double a){
a *= a;
return a;
}

int main(void){
double x1,y1,x2,y2,sum;
double len;

scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);

sum = D(x1 - x2) + D(y1 - y2);
len = sqrt(sum);

printf("%f\n",len);
return 0;
}