How can I create a IJVM-Code to calculate the factorial of 5?

Question

I have the following problem for which I cannot find a solution:

Calculate the factorial n! of a given number n ∈ N. To do this, extend the file fak.jvm by implementing the method fak (without adding further method calls). method calls). You may use local variables (but it also works without). The result should be output at the end in the following form, here exemplary for n = 5:

Result: fak(5) = 120 To do this, implement the following function in IJVM:

int fak (int a) {
   int result = 1;
   while ( a > 1) {
      result ∗= a;
      a−−;
   }
   return result;
}

I have tried this, but it's not working:

; main-Methode
 BIPUSH 0
 BIPUSH 5                               // n
 INVOKEVIRTUAL fac
 
 ; print
 SPRINT "Result: fac("
 BIPUSH 5                               // n
 IPRINT
 SPRINT ") = "
 IPRINT
 SPRINT "\n"
 HALT


fac 2 1
 ILOAD 1
 BIPUSH 1
 IF_ICMPEQ end_loop
 
 BIPUSH 0
 ILOAD 1
 ILOAD 1
 BIPUSH 1
 ISUB
 INVOKEVIRTUAL fac
 IMUL
 IRETURN
end_loop:
 BIPUSH 1
 IRETURN

When I run the code I get "Result: fac(5)=0". But I would like to have "Result: fac(5)=120". Does anyone know what I did wrong?