参考代码:

#include <bits/stdc++.h>

using namespace std;

int main()

{

    double xa,ya,xb,yb;

    cin>>xa>>ya>>xb>>yb;

    double c=sqrt((xa-xb)*(xa-xb)+(ya-yb)*(ya-yb));

    printf("%.3lf",c);

    return 0;

}