C# dynamic keyword not dynamic as i think !!!!!!

//I have this method that use dynamic
public static dynamic Fact(dynamic r)
{
    dynamic result = 1;
    for (dynamic i = 1; i <= r; i++)
    {
        result *= i;
    }
    return result;
}
//I call it in main as
static void Main(string[] args)
{
    for (int i = 9; i < 100; i++)
    {
        dynamic x=helper_method.Fact(i);
        Console.WriteLine("{0}\t>>{1}\t{2}", i, x,x.GetType());
    }
}
//the output type is Int32 at all
//I think since it is dynamic the type will changed when match maximum int32
//and dynamic will automatically change type Int32 to Int64

//what is the benefit of dynamic if it's type not changed at runtime?????????

//output here
/*
9       >>362880        System.Int32
10      >>3628800       System.Int32
11      >>39916800      System.Int32
12      >>479001600     System.Int32
13      >>1932053504    System.Int32
14      >>1278945280    System.Int32
15      >>2004310016    System.Int32
16      >>2004189184    System.Int32
17      >>-288522240    System.Int32
18      >>-898433024    System.Int32
19      >>109641728     System.Int32
20      >>-2102132736   System.Int32
21      >>-1195114496   System.Int32
22      >>-522715136    System.Int32
23      >>862453760     System.Int32
24      >>-775946240    System.Int32
25      >>2076180480    System.Int32
26      >>-1853882368   System.Int32
27      >>1484783616    System.Int32
28      >>-1375731712   System.Int32
29      >>-1241513984   System.Int32
30      >>1409286144    System.Int32
31      >>738197504     System.Int32
32      >>-2147483648   System.Int32
33      >>-2147483648   System.Int32
34      >>0     System.Int32
35      >>0     System.Int32
36      >>0     System.Int32
37      >>0     System.Int32
38      >>0     System.Int32
39      >>0     System.Int32
40      >>0     System.Int32
41      >>0     System.Int32
42      >>0     System.Int32
43      >>0     System.Int32
44      >>0     System.Int32
45      >>0     System.Int32
46      >>0     System.Int32
*/