diff --git a/index.js b/index.js
index 3bb4d07..a3b5348 100644
--- a/index.js
+++ b/index.js
@@ -4,7 +4,51 @@
  * Depending on the operation, either add up all of the numbers or subtract all of the numbers, from left to right.
  * @returns {number} The result of either adding all numbers or subtracting all numbers, depending on the arguments added to the command line.
  */
-function calculator() {}
+function calculator() {
+	// 1. Create a variable to hold the final result of the operation.
+	let result = 0;
+	//  2. `accumulator`
+	let num = 0;
+	// 3. another `accumulator` for the first element
+	let firstNum = Number(process.argv[3]);
+
+	// 4. Create a variable that accesses the inputted operation.
+	const operation = process.argv[2];
+
+	// 5. Error handling using guard clauses
+	if (operation === undefined) {
+		result = 'No operation provided...';
+	}
+	if (operation && process.argv[3] === undefined) {
+		result = 'No numbers provided...';
+	}
+	if (
+		operation &&
+		process.argv[3] &&
+		(operation !== 'plus' || operation !== 'minus')
+	) {
+		result = `Invalid operation: ${operation}`;
+	}
+
+	// 6. Iterate over the `process.argv` array.
+	for (let i = 4; i < process.argv.length; i++) {
+		// 7. If the operation passed in is `plus`, add the numbers together sequentially using the `accumulator pattern` and reassign the total to `result`.
+		if (operation === 'plus') {
+			// 8. Convert the inputted string to a `number` data type to be able to perform arithmetic operations on.
+			num = Number(process.argv[i]);
+			firstNum += num;
+			result = firstNum;
+		}
+		// 9. If the operation passed in is `minus`, subtract the numbers sequentially using the `accumulator pattern` and reassign the result to `result`.
+		if (operation === 'minus') {
+			num = Number(process.argv[i]);
+			firstNum -= num;
+			result = firstNum;
+		}
+	}
+
+	return result;
+}
 
 // Don't change anything below this line.
 module.exports = calculator;
diff --git a/run.js b/run.js
index d39acfd..76e02ea 100644
--- a/run.js
+++ b/run.js
@@ -1,3 +1,3 @@
-const calculator = require("./");
+const calculator = require('./');
 const result = calculator();
 console.log(result);