What is printed when the following program is executed?

public class TailSum {
    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(5, 0));
    }
}