diff --git a/02a.lisp b/02a.lisp
new file mode 100644
index 0000000..abf5fc0
--- /dev/null
+++ b/02a.lisp
@@ -0,0 +1,47 @@
+(require :uiop)
+(load (sb-ext:posix-getenv "ASDF"))
+(asdf:load-system 'str)
+
+(defun get-range (range)
+  (let ((range (str:split "-" range)))
+    (loop for i from (parse-integer (car range)) to (parse-integer (car (cdr range))) collect i)
+    )
+  )
+(get-range "11-22")
+
+(defun get-ranges (instr)
+  (loop for range in
+        (str:split "," (string-trim '(#\Newline) instr))
+        collect
+        (get-range range)
+    )
+  )
+(get-ranges "1-3,4-5")
+
+(defun validate-product-id (product-id)
+  (let ((product-id (format nil "~A" product-id)))
+    (setq product-id-length (length product-id))
+    (setq split-at (/ product-id-length 2))
+    ;(print product-id)
+    (cond
+       ((not (evenp product-id-length)) t)
+       (t (not (string= (subseq product-id 0 split-at) (subseq product-id split-at))))
+      )
+    )
+  )
+(validate-product-id 1212)
+(validate-product-id 123)
+(validate-product-id 1234)
+
+(defun sum-all-invalid-ids (text)
+  (reduce #'+
+    (remove-if #'validate-product-id
+      (reduce #'append
+        (get-ranges text)
+        )
+      )
+    )
+  )
+
+(sum-all-invalid-ids (uiop:read-file-string "02example.txt"))
+(sum-all-invalid-ids (uiop:read-file-string "02data.txt"))
diff --git a/02example.txt b/02example.txt
new file mode 100644
index 0000000..a3f22ef
--- /dev/null
+++ b/02example.txt
@@ -0,0 +1 @@
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124
diff --git a/flake.lock b/flake.lock
new file mode 100644
index 0000000..31a72ac
--- /dev/null
+++ b/flake.lock
@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1764522689,
+        "narHash": "sha256-SqUuBFjhl/kpDiVaKLQBoD8TLD+/cTUzzgVFoaHrkqY=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "8bb5646e0bed5dbd3ab08c7a7cc15b75ab4e1d0f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-25.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 0000000..a47cafa
--- /dev/null
+++ b/flake.nix
@@ -0,0 +1,32 @@
+{
+  description = "A Nix-flake-based Shell development environment";
+
+  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-25.11"; # unstable Nixpkgs
+
+  outputs = {self, ...} @ inputs: let
+    supportedSystems = [
+      "x86_64-linux"
+      "aarch64-linux"
+      "x86_64-darwin"
+      "aarch64-darwin"
+    ];
+    forEachSupportedSystem = f:
+      inputs.nixpkgs.lib.genAttrs supportedSystems (
+        system:
+          f {
+            pkgs = import inputs.nixpkgs {inherit system;};
+          }
+      );
+  in {
+    devShells = forEachSupportedSystem (
+      {pkgs}: {
+        default = pkgs.mkShellNoCC {
+          packages = with pkgs; [
+            shellcheck
+            (sbcl.withPackages (ps: [ps.str]))
+          ];
+        };
+      }
+    );
+  };
+}