Simple Calculator program using Java

import java.util.Scanner;

public class calculator

{
public static void main(String[] args)
{
char answer;
String input;
int firstnumber;
int secondnumber;
System.out.println ("Welcome to the calculator!");
System.out.println ("To multiply use m, to divide use d, to add use a, to subtract use S");
Scanner keyboard = new Scanner(System.in);

do
{
System.out.println ("Do you want to multiply, divide, add, or subtract? ");
keyboard.nextLine();
input = keyboard.nextLine();
answer = input.charAt(0);
if (answer == 'm')
System.out.println ("Input first number: ");
keyboard.nextLine();
input = keyboard.nextLine();
firstnumber = keyboard.nextInt();
System.out.println ("Would you like another calculation (y/n)? ");
keyboard.nextLine();
input = keyboard.nextLine();
answer = input.charAt(0);
}

while(answer == 'y')
if(answer == 'n');
System.exit(0);
}
}