Recursive Method Return Value

Consider the following code segment. What value is returned by the method call tailSum(4, 0)?

public class Main {
    public static int tailSum(int n, int accumulator) {
        if(n == 0)
            return accumulator;
        return tailSum(n - 1, accumulator + n);
    }
    public static void main(String[] args) {
        System.out.println(tailSum(4, 0));
    }
}