#include <cstdio>
#include <cmath>
int main(){
    double x1,y1,x2,y2;
    scanf("%lf%lf",&x1,&y1);
    scanf("%lf%lf",&x2,&y2);
    double d=sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));//先输入才能计算，不然最后的结果是0，注意语句是有顺序的
    printf("%.4lf",d);
    return 0;
}