#include<bits/stdc++.h>
using namespace std;
int main()
{
	double xa,ya,xb,yb;
	cin>>xa>>ya>>xb>>yb;
	double a=xa-xb;
	double b=ya-yb;
	double c=sqrt(a*a+b*b);
	cout<<fixed<<setprecision(3)<<c;
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
int main()
{
	double xa,ya,xb,yb;
	cin>>xa>>ya>>xb>>yb;
	double a=xa-xb;
	double b=ya-yb;
	double c=sqrt(a*a+b*b);
	cout<<fixed<<setprecision(3)<<c;
	return 0;
}

这叫做实力！！！！

#include<bits/stdc++.h> using namespace std; int main() { double xa,ya,xb,yb; cin>>xa>>ya>>xb>>yb; double a=abs(xa-xb); double b=abs(ya-yb); double c=sqrt(aa+bb); cout<<fixed<<setprecision(3)<<c; return 0; }

666