Cho e hỏi lỗi unresolved external symbol _main referenced in function "int __cdecl invoke_main(void)" là s ạ?

chương trình cộng 2 ma trận

#include<iostream>
#include<iomanip>
#include<stdio.h>
#include<conio.h>
using namespace std;	
int AllocMatrix(int **a, int n, int m)
{
	*a= new int [n*m];
	if (*a==NULL) return 0;
	return 1;
}
void FreeMatrix(int *a)
{
	if(a!=NULL)
	delete [] a ;
}

void Input(int *a,int n, int m, char S)
{
    a=new int [n*m];
    
	for(int i=0;i<n;++i)
		for(int j=0;j<m;++j)
			
	{
		cout<<S<<"["<<i+1<<"]["<<j+1<<"]=";
		cin>>a[i*m+j];
	
    }
}
void Output(int *a, int n, int m)
{ 
    int cnt=0;
	for (int i=0;i<n;++i)
	  for (int j=0;j<m;++j)
	{
	cout<<setw(3)<<a[i*m+j];
		cnt++;
		if (cnt%m==0)
		cout<<endl;
	}	
}	
void AddMatrix(int *a, int *b, int *c, int n, int m)
{
	int i;
	a=new int[n*m];
    b=new int [n*m];
    c=new int [n*m];
	
	for( i=0;i<n*m;++i)
     c[i]=a[i] + b[i];
}	

int main()
{
	int n,m,i,j,cnt;
    int *a=NULL,*b=NULL,*c=NULL;
    AllocMatrix(&a,n,m);
	if(!AllocMatrix(&a,n,m))
		{
	cout<<"khong con du bo nho"<<endl;
       return 1;}
    AllocMatrix(&b,n,m);   
    if (!AllocMatrix(&b,n,m))
       {
        cout<<"khong con du bo nho"<<endl;
        FreeMatrix(a);
       return 1;}
    AllocMatrix(&c,n,m);   
	if (!AllocMatrix(&c,n,m))
   	  {
   	    cout<<"khong con du bo nho"<<endl;
        FreeMatrix(a);
        FreeMatrix(b);
       return 1;}
	cout<<"nhap so hang cua ma tran n=";cin>>n;cout<<endl;
	cout<<"nhap so cot cua ma tran m=";cin>>m;cout<<endl;
    a=new int [n*m];
	cout<< "nhap ma tran thu nhat " << endl;  
    Input(a,n,m,'a');
    b=new int [n*m];
    cout<<"nhap ma tran thu hai "<< endl;
    Input(b,n,m,'b');
    c=new int [n*m];
    AddMatrix(a,b,c,n,m);
    cout<<"ma tran thu nhat la "<<endl;
    Output(a,n,m);
    cout<<"ma tran thu hai la "<<endl;
    Output(b,n,m);
    cout<<"tong cua hai ma tran la"<<endl;	
	Output(c,n,m);
	FreeMatrix(a);
	FreeMatrix(b);
	FreeMatrix(c);
	return 0;
	getch();
}