Click on random button/link

I have problem. I want to random click on button "add to cart" Here is a website: demo.opencart -compare

Im not sure what I do wrong- my code:

List <WebElement> links = driver.findElements(By.xpath("//input[@value = \"Add to Cart\"]"));
            Random gen = new Random();
            WebElement link = links.get(gen.nextInt(links.size()));

            link.click();

and i have error:

java.lang.IllegalArgumentException: bound must be positive
    at java.util.Random.nextInt(Random.java:388)
    at pages.HomePage.chooseRandomItem(HomePage.java:112)
    at testes.MainPage.chooseRandomItemToCart(MainPage.java:62)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:86)
    at org.testng.internal.Invoker.invokeMethod(Invoker.java:643)
    at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:820)
    at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1128)
    at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:129)
    at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:112)
    at org.testng.TestRunner.privateRun(TestRunner.java:782)
    at org.testng.TestRunner.run(TestRunner.java:632)
    at org.testng.SuiteRunner.runTest(SuiteRunner.java:366)
    at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:361)
    at org.testng.SuiteRunner.privateRun(SuiteRunner.java:319)
    at org.testng.SuiteRunner.run(SuiteRunner.java:268)
    at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
    at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
    at org.testng.TestNG.runSuitesSequentially(TestNG.java:1244)
    at org.testng.TestNG.runSuitesLocally(TestNG.java:1169)
    at org.testng.TestNG.run(TestNG.java:1064)
    at org.testng.remote.RemoteTestNG.run(RemoteTestNG.java:113)
    at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:206)
    at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:177

I would like to ask U to help me