题解

#include<bits/stdc++.h>
using namespace std;
int main()
{
	double x1,y1,x2,y2,x3,y3;
	cin>>x1>>y1>>x2>>y2>>x3>>y3;
	double s = sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
	s = s+sqrt((x2-x3)*(x2-x3)+(y2-y3)*(y2-y3));
    s = s+sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
	cout<<fixed<<setprecision(2)<<s;
	return 0;
}