蒟蒻求解：为什么这段代码爆0了？

代码如下：

#include<iostream>
#include<cstdio>
#include<cmath>

using namespace std;

double discover(double x1,double x2,double y1,double y2){
    double dis;
    dis=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
    return dis;
}
int main(){
    double a1,b1,a2,b2,a3,b3,sum=0;
    cin >> a1 >> b1 >> a2 >> b2 >> a3 >> b3;
    sum+=discover(a1,a2,b1,b2);
    sum+=discover(a1,a3,b1,b3);
    sum+=discover(a2,a3,b2,b3);
    printf("%2lf",sum);
    return 0;
}

J某百思不得其解，只好求助各位神犇。在此感激不尽