求助！！！为什么会CE

#include<iostream>
#include<iomanip>
#include<algorithm>
#include<cmath>
using namespace std;
struct pi{
	int x,y,z;
}a[50001];
bool cmp(pi x,pi y){
	return x.z<y.z;
}
double dis(int x1,int x2,int y1,int y2,int z1,int z2){
	double x=pow(x2-x1,2);
	double y=pow(y2-y1,2);
	double z=pow(y2-y1,2);
	return sqrt(x+y+z);
}
int main(){
	int n;
	double ans=0;
	cin>>n;
	for(int i=1;i<=n;i++)cin>>a[i].x>>a[i].y>>a[i].z;
	sort(a+1,a+n+1,cmp);
	for(int i=2;i<=n;i++){
		ans+=dis(a[i-1].x,a[i].x,a[i-1].y,a[i].y,a[i-1].z,a[i].z);
	}
	cout<<fixed<<setprecision(3)<<ans;
	return 0;
}