diff --git a/lesson#4/main.d b/lesson#4/main.d
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+++ b/lesson#4/main.d
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+//	LET'S CREATE A CALCULATOR!
+//	LEARN: USER INPUT, IF STATEMENTS (logical expressions), ARITHMETIC OPERATIONS
+
+//	importing the entire library
+//	import std.stdio;
+
+//	importing writeln, write and readf functions
+import std.stdio: writeln, write, readf;
+
+void main() {
+	float a = 0;
+	float b = 0;
+	char operation; //	'+', '-', '*', '/'
+
+	write("first number: ");
+	readf(" %s", a);
+
+	write("second number: ");
+	readf(" %s", b);
+
+	write("operation: ");
+	readf(" %s", operation);
+
+	/*
+	//	readf could be used to read everything at once
+	write("expression: ");
+	readf(" %s %s %s", a, operation, b);
+	*/
+
+	/*	
+		Boolean data type, can either be true or false. 
+		bool gameOver = false;
+
+		The syntax of an if statement
+		if(condition) {
+			instructions;
+		} else {
+			instructions;
+		}
+
+		if(condition1) {
+			instructions;
+		} else if(condition2) {
+			instructions;
+		} else if(...) {
+			instructions;
+		} else {
+			instructions
+		}
+	*/
+
+	if(operation == '+') {
+		writeln(a, operation, b, "=", a+b);
+	} else if(operation == '-') {
+		writeln(a, operation, b, "=", a-b);
+	} else if(operation == '*') {
+		writeln(a, operation, b, "=", a*b);
+	} else if(operation == '/') {
+		writeln(a, operation, b, "=", a/b);
+	} else {
+		writeln("Unknown operation! Error!");
+	}
+
+	/*
+		== (is equal to), != (is not equal to), && (and), || (or)
+
+		operation could either be '+' or '-'
+		if(operation == '+' || operation == '-') { 
+			instructions;
+		}
+
+		we check if both a and b are 0
+		if(a == 0 && b == 0) {
+			instructions;
+		}
+	*/
+}
+
+
+