add some more checks in main file for correctness

main.cpp

@@ -33,6 +33,15 @@ int main() {
     add_rect(2,2,7,3);
     check_area(52);
 
+    add_rect(-1,2,5,7);
+    check_area(68);
+
+    add_rect(-1,0,8,4);
+    check_area(70);
+
+    add_rect(3,3,7,5);
+    check_area(71);
+
     std::cout << "All checks passed" << std::endl;
     return 0;
 }