如下

#include <bits/stdc++.h>

using namespace std;
int a1,a2,b1,b2,c1,c2;
double a;
int jisuan(int a,int aa,int b,int bb){//sqrt[(b-a)*(b-a)+(bb-aa)*(bb-aa)]
    double q=abs(sqrt(abs((b-a)*(b-a))+abs((bb-aa)*(bb-aa))));
    return q;
}
int main()
{
    freopen("P5735.in","r",stdin);
    freopen("P5735.out","w",stdout);
    cin>>a1>>a2;
    cin>>b1>>b2;
    cin>>c1>>c2;
    a=abs(jisuan(a1,a2,b1,b2))+abs(jisuan(a1,a2,c1,c2))+abs(jisuan(b1,b2,c1,c2));
    printf("%0.2lf",a);
    fclose(stdin);
    fclose(stdout);
}