How to increment years

My program works perfectly except for one thing. Lets say I enter 3 years in the input box. The months change 1, 2, 3… but how do I change the year from 1 to 2 to 3 after each 12 month period? Currently, it just stays 1.

import java.util.Scanner;
import javax.swing.JOptionPane;

public class AverageRainfall {
    public static void main(String[] args) {
        int y = 1;
        int num_of_years = 0;
        int total_num_of_months = 0;
        int NUM_OF_MONTHS = 12;
        double rainfall = 0;
        double total_rainfall = 0;
        double average_rainfall_per_month = 0;
        String input;

        input = JOptionPane.showInputDialog("Enter the number of years: ");
        num_of_years = Integer.parseInt(input);

        while (num_of_years < 1){
            input = JOptionPane.showInputDialog("Invalid. Enter 1 or greater: ");
            num_of_years = Integer.parseInt(input);
        }
        for (int i = 0; i < num_of_years; i++){
            for(int j = 0; j < NUM_OF_MONTHS; j++){
                input = JOptionPane.showInputDialog("Enter the rainfall, in inches, for each month." + "\n"
                        + "Year " + (y + 1/12) + " Month " + (j + 1) + ":");
                rainfall = Double.parseDouble(input);
                while (rainfall < 0){
                    input = JOptionPane.showInputDialog("Invalid. Enter 0 or greater: ");
                    rainfall = Double.parseDouble(input);
                }
                total_rainfall += rainfall;
                total_num_of_months++;
            }
        }
        average_rainfall_per_month = total_rainfall / total_num_of_months;
        System.out.println("Total number of months: " + total_num_of_months);
        System.out.println("Total inches of rainfall: " + total_rainfall);
        System.out.println("Average rainfall per month = " + average_rainfall_per_month);
    }
}

>Solution :

        ...
        total_num_of_months++;
    }
    //->increment year variable here
}