diff --git a/.gitignore b/.gitignore
index 7347638..075f737 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,9 @@
 .AppleDouble
 .LSOverride
 
+*.cmi
+*.cmo
+
 # Icon must end with two \r
 Icon

 
diff --git a/README.md b/README.md
index c5b1f00..0fbe252 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,14 @@ OCaml Language tests
 
 https://ocaml.org/
 
+<br>
+
+Compile single file:
+```shell
+$ ocamlc -o gcd gcd.ml
+```
+
+<br>
 
 Install on macOS
 ```shell
diff --git a/gcd.ml b/gcd.ml
new file mode 100644
index 0000000..96708bf
--- /dev/null
+++ b/gcd.ml
@@ -0,0 +1,18 @@
+(* File gcd.ml *)
+let rec gcd a b =
+  if b = 0 then a
+  else gcd b (a mod b);;
+
+let main () =
+  let len = Array.length Sys.argv in
+  Printf.printf "array length %d\n" len;
+  if len < 3 then (
+    Printf.printf "must have 2 args";
+    exit 0;
+  );
+  let a = int_of_string Sys.argv.(1) in
+  let b = int_of_string Sys.argv.(2) in
+  Printf.printf "gcd of %d and %d is %d\n" a b (gcd a b);
+  exit 0;;
+main ();;
+