Added solution

src/main/java/com/company/ServiceImpl.java

@@ -2,18 +2,18 @@
 
 public class ServiceImpl implements Service {
     public int firstMethod(int a, int b) {
-        return 0;
+        return a + b;
     }
 
     public int secondMethod(int a, int b) {
-        return 0;
+        return a - b;
     }
 
     public int thirdMethod(int a, int b) {
-        return 0;
+        return a * b;
     }
 
     public int fourthMethod(int a, int b) {
-        return 0;
+        return a / b;
     }
 }

variant.txt

@@ -0,0 +1 @@
+1