Sao phần xoa_DS của e có gì sai hả mn

#include<stdio.h>
#include<conio.h>
#include<iostream>
#include<string.h>
using namespace std;
struct NGSINH
{
	int ngay,thang,nam;
};
struct HOCSINH
{
	char sbd[6];
	int gtinh;
	char hoten[20];
	char hkhau[20];
	NGSINH	ngsinh;
	float tongdiem;
};
void nhap(HOCSINH a[20],int n)
{
	int i;
	
	for(i=1;i<=n;i++)
		{
			cout<<"Nhap vao thong tin hoc sinh "<<i<<":"<<endl;
			fflush(stdin);
			cout<<"		So bao danh: ";					gets(a[i].sbd);
			fflush(stdin);
			cout<<endl<<"		Gioi tinh: ";				cin>>a[i].gtinh; // 1:nam 2:nu
			
			fflush(stdin);
			
			cout<<endl<<"		Nhap vao ho va ten: ";		gets(a[i].hoten);	
			
			fflush(stdin);
			
			cout<<endl<<"		Nhap vao ho khau: ";		gets(a[i].hkhau);		
			
			fflush(stdin);
			cout<<endl<<"		Nhap vao ngay sinh: "<<endl;
			cout<<"		Ngay: ";							cin>>a[i].ngsinh.ngay;
			cout<<endl<<"		Thang: ";					cin>>a[i].ngsinh.thang;
			cout<<endl<<"		Nam: ";					cin>>a[i].ngsinh.nam;
			cout<<endl<<"		Nhap vao tong diem: "; 	cin>>a[i].tongdiem;
		}
}
void in(HOCSINH a[20],int n)
{
	int i;
	cout<<"Danh sach hoc sinh: ";
	for(i=1;i<=n;i++)
		{
			cout<<a[i].hoten<<endl;
		}
}
void in_dk(HOCSINH a[20],int n)
{
	int i;
	cout<<endl<<"hoc sinh nu co tong diem lon hon 25 la: "<<endl;
	for(i=1;i<=n;i++)
		{
			if(a[i].gtinh==2&&a[i].tongdiem>25)
				cout<<a[i].hoten<<endl;
		}
}
float trung_Tuyen(HOCSINH a[20],int n)
{
	int i,dem=0;
	float diem;
	cout<<"Nhap vao diem chuan: ";			cin>>diem;
	for(i=1;i<=n;i++)
		{
			if(a[i].tongdiem>diem)
				{
					dem++;
					if(strcmp(a[i].hkhau,"HA NOI")==0)
						{
							cout<<a[i].sbd;
							cout<<a[i].gtinh;
							cout<<a[i].hoten;
							cout<<a[i].hkhau;
							cout<<a[i].ngsinh.ngay<<"-"<<a[i].ngsinh.thang<<"-"<<a[i].ngsinh.nam;
							cout<<a[i].tongdiem;
						}	
				}
				
		}
		return dem;
}
void xoa_DS(HOCSINH a[20],int n)
{
	int i,k;
	for(i=1;i<=n;i++)
		{
			if(strcmp(a[i].sbd,"DCNA2")==0)
				{
					
					for(k=i;k<=n;k++)
						{
							a[k].hoten=a[k+1].hoten;
						}
				n--;
				}
			
		}
	cout<<"Danh sach hoc sinh sau khi xoa: "<<endl;
	for(i=1;i<=n;i++)
	{
		cout<<a[i].hoten<<endl;
	}
	
}
main()
{	int n,kq;
	
	do{
		cout<<"Nhap vao so hoc sinh: ";
		cin>>n;
	}while(n<2||n>20);
	HOCSINH a[n];
	nhap(a,n);
	in(a,n);
	in_dk(a,n);
	
	kq=trung_Tuyen(a,n);
	cout<<"Co "<<kq<<"thi sinh trung tuyen. ";
	xoa_DS(a,n);
}