diff --git a/.gitignore b/.gitignore
index 664efa7..a5744ba 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
 *cache*
 .coverage
 .idea
+
+.env
\ No newline at end of file
diff --git a/Makefile b/Makefile
index 3c23d42..1b48145 100644
--- a/Makefile
+++ b/Makefile
@@ -2,5 +2,5 @@ dev:
 	pip install -U -r requirements.txt
 
 test:
-	flake8 .
+	flake8 . --exclude .env
 	py.test --cov=. .
\ No newline at end of file
diff --git a/calculator.py b/calculator.py
index 0e91157..a3509b3 100644
--- a/calculator.py
+++ b/calculator.py
@@ -29,3 +29,7 @@ def add(self, x: float, y: float) -> float:
             float : The sum of two given numbers.
         """
         return x + y
+
+    def minus(self, x: float, y: float) -> float:
+        """Return the minus of two inputs."
+        pass
diff --git a/requirements.txt b/requirements.txt
index 67dbbac..32af406 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,5 @@
-flake8
-flake8-bugbear
-flake8-docstrings
-pytest
-pytest-cov
\ No newline at end of file
+flake8 == 3.7.7
+flake8-bugbear == 18.8.0
+flake8-docstrings == 1.3.0
+pytest == 4.2.0
+pytest-cov == 2.6.1
\ No newline at end of file