diff --git a/build.gradle b/build.gradle
index 8121a72..6232e94 100644
--- a/build.gradle
+++ b/build.gradle
@@ -16,4 +16,4 @@ dependencies {
 
 test {
     useJUnitPlatform()
-}
\ No newline at end of file
+}
diff --git a/settings.gradle b/settings.gradle
index fc7105d..eb3564d 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -1,2 +1 @@
 rootProject.name = 'parallel-programming'
-
diff --git a/src/main/java/ru/lionarius/AreaCalculator.java b/src/main/java/ru/lionarius/AreaCalculator.java
new file mode 100644
index 0000000..2e1f1c1
--- /dev/null
+++ b/src/main/java/ru/lionarius/AreaCalculator.java
@@ -0,0 +1,11 @@
+package ru.lionarius;
+
+public class AreaCalculator {
+
+    public double circle(double radius) {
+        if (radius < 0.0)
+            throw new IllegalArgumentException("radius cannot be negative");
+
+        return Math.PI * radius * radius;
+    }
+}
diff --git a/src/main/java/ru/lionarius/Main.java b/src/main/java/ru/lionarius/Main.java
index f40e1bf..8023c11 100644
--- a/src/main/java/ru/lionarius/Main.java
+++ b/src/main/java/ru/lionarius/Main.java
@@ -4,4 +4,4 @@ public class Main {
     public static void main(String[] args) {
         System.out.println("Hello world!");
     }
-}
\ No newline at end of file
+}
diff --git a/src/test/java/AreaCalculatorTests.java b/src/test/java/AreaCalculatorTests.java
new file mode 100644
index 0000000..615a8ac
--- /dev/null
+++ b/src/test/java/AreaCalculatorTests.java
@@ -0,0 +1,21 @@
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+import ru.lionarius.AreaCalculator;
+
+class AreaCalculatorTests {
+
+    private final AreaCalculator areaCalculator = new AreaCalculator();
+
+    @Test
+    void circleArea() {
+        assertEquals(0.0, this.areaCalculator.circle(0.0));
+        assertEquals(Math.PI, this.areaCalculator.circle(1.0));
+        assertEquals(Math.PI * 4.0, this.areaCalculator.circle(2.0));
+
+        assertThrowsExactly(IllegalArgumentException.class, () -> this.areaCalculator.circle(-1.0));
+
+        assertEquals(Double.POSITIVE_INFINITY, this.areaCalculator.circle(Double.MAX_VALUE));
+        assertEquals(Double.NaN, this.areaCalculator.circle(Double.NaN));
+    }
+}