diff --git a/bin/main.ml b/bin/main.ml
index ee3e4f3..0dc832d 100644
--- a/bin/main.ml
+++ b/bin/main.ml
@@ -10,7 +10,7 @@ let program =
     )
 ;;
 
-Printf.printf "%d\n" (reduce 100 program)
+Printf.printf "%d\n" (reduce 1 program)
 
 (* -------------------------------------------------------------------------- *)
 (* y not defined program *)
@@ -57,7 +57,7 @@ Printf.printf "%d\n" (reduce 10 program)
 ;;
 
 (* -------------------------------------------------------------------------- *)
-(* hailstone sequence lenght program *)
+(* hailstone sequence's lenght program *)
 let program =
   Main
     ("a",
diff --git a/lib/miniImp.ml b/lib/miniImp.ml
index 773834e..6a8e507 100644
--- a/lib/miniImp.ml
+++ b/lib/miniImp.ml
@@ -28,6 +28,7 @@ and a_exp =
   | Division of a_exp * a_exp            (* a / a *)
   | Modulo of a_exp * a_exp              (* a % a *)
   | Power of a_exp * a_exp               (* a ^ a *)
+  | PowerMod of a_exp * a_exp * a_exp    (* a ^ a % a *)
   | Rand of a_exp                        (* rand(0, a) *)
 
 
@@ -118,6 +119,12 @@ and evaluate_a (mem: memory) (exp_a: a_exp) =
       let exp_a2val = evaluate_a mem exp_a2 in
       Utility.pow exp_a1val exp_a2val
     )
+  | PowerMod (exp_a1, exp_a2, exp_a3) -> (
+      let exp_a1val = evaluate_a mem exp_a1 in
+      let exp_a2val = evaluate_a mem exp_a2 in
+      let exp_a3val = evaluate_a mem exp_a3 in
+      Utility.powmod exp_a1val exp_a3val exp_a2val
+    )
   | Rand (exp_a) -> (
       Random.int (evaluate_a mem exp_a)
     )
diff --git a/lib/miniImp.mli b/lib/miniImp.mli
index 49cf20b..183c285 100644
--- a/lib/miniImp.mli
+++ b/lib/miniImp.mli
@@ -28,6 +28,7 @@ and a_exp =
   | Division of a_exp * a_exp            (* a / a *)
   | Modulo of a_exp * a_exp              (* a % a *)
   | Power of a_exp * a_exp               (* a ^ a *)
+  | PowerMod of a_exp * a_exp * a_exp    (* a ^ a % a *)
   | Rand of a_exp                        (* rand(0, a) *)
 
 
diff --git a/lib/utility.ml b/lib/utility.ml
index 6b979ec..d16e044 100644
--- a/lib/utility.ml
+++ b/lib/utility.ml
@@ -4,3 +4,10 @@ let rec pow a = function
   | n ->
     let b = pow a (n / 2) in
     b * b * (if n mod 2 = 0 then 1 else a)
+
+let rec powmod a d = function
+  | 0 -> 1
+  | 1 -> a mod d
+  | n ->
+    let b = (powmod a d (n / 2)) mod d in
+    (((b * b) mod d) * (if n mod 2 = 0 then 1 else a)) mod d
diff --git a/lib/utility.mli b/lib/utility.mli
index d00d6c4..c5f2cfb 100644
--- a/lib/utility.mli
+++ b/lib/utility.mli
@@ -1 +1,3 @@
 val pow : int -> int -> int
+
+val powmod : int -> int -> int -> int