Multy Dim Array

Question

I need a multi dimensional (Jaged Array) for entering Matrix data. Can any body telll whats wrojng with the code I have attched here...

 a=new int[m][];
      for(i=0;i<m;i++)
      {
 
 Console.WriteLine("No. Of Cols: in Raw "+(i+1));
       n=Convert.ToInt32(Console.ReadLine()); 
          
 for (j=0;j<n;j++)
         { 
  Console.WriteLine("Enter cl: "+(j+1));
         a[i][j]=Convert.ToInt32(Console.ReadLine());
         } 
      }

      for( i=0;i<a.Length;i++)
      {
    for ( j=0;j<a[i].Length -1;j++)
         {
         Console.Write(a[i][j]+"\t");
         } 
 Console.WriteLine("\n"); 
      }

  }
}

Error Message while executing the progrma:"Unhandled Exception: System.NullReferenceException: Object reference not set to
an instance of an object.
   at MyArrayc2.Main()"