What is the error about unsafe code?

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C#

C#4.0

Error 1 Unsafe code may only appear if compiling with /unsafe C:\Visual Studio 2010\Projects\Unsafe\Unsafe\Program.cs 6 24 Unsafe
Error 2 Unsafe code may only appear if compiling with /unsafe C:\Visual Studio 2010\Projects\Unsafe\Unsafe\Program.cs 10 31 Unsafe

using System;

class Unsafe
{
    unsafe static void SquarePtrParam(int *p)
    {
        *p *= *p;
    }

    unsafe public static void Main(string[] args)
    {
        int i = 5;
        SquarePtrParam(&i);
        Console.WriteLine(i);
    }
}

[EMAIL REMOVED] Thanks.