为什么这样写提交会编译失败,求大佬解释?

#include<bits/stdc++.h>
using namespace std;
const int B=0;
double x1,y1,x2,y2,x3,y3,ans;
void dis(double x_1,double y_1,double x_2,double y_2){
    double t=pow(abs(x_2-x_1),2)+pow(abs(y_2-y_1),2);
    ans=ans+sqrt(t);
} 
int main(){
    cin>>x1>>y1>>x2>>y2>>x3>>y3;
    dis(x1,y1,x2,y2);
    dis(x1,y1,x3,y3);
    dis(x2,y2,x3,y3);
    printf("%.2lf",ans);

    return 0;
}
/*
0 0
0 3
4 0
*/