样例过但是0分

#include<bits/stdc++.h>
using namespace std;
double dis(double x1,double y1,double x2,double y2){
       double sum;
       sum=sqrt((x2-x1)*(x2-x1)*(y2-y1)*(y2-y1));
       return sum;
}
int main(){
    double x1,x2,x3,y1,y2,y3,ab,ac,bc,a;
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    ab=dis(x1,y1,x2,y2);
    bc=dis(x2,y2,x3,y3);
    ac=dis(x1,y1,x3,y3);
    a=ab+ac+bc;
    printf("%.2f",a);
}