RT?究竟怎么错的？？？

#include<bits/stdc++.h>
using namespace std;

double f(double a1,double a2,double b1,double b2){
     
    double x=a1-b1;
    double y=a2-b2;
     
    double as=sqrt(x*x+y*y);
     
    return as;
}

int main(){
    
	double sum=0,a[3][2];
    
	for(int i=0;i<3;i++){
        
		cin>>a[i][0]>>a[i][1];
    }
    
    sum+=f(a[0][0],a[0][1],a[1][0],a[1][1]);
    sum+=f(a[2][0],a[0][1],a[1][0],a[1][1]);
    sum+=f(a[0][0],a[0][1],a[2][0],a[2][1]);
    
    cout<<sum<<setprecision(3);
    
    return 0;
}