Out Of Bounds

import java.util.*;

public class Calculator
{
  public char hp;
  public int finalStat;
  public double natureMult;

  public void readInput()
  {
    Scanner keys = new Scanner(System.in);

    System.out.println("Enter the base stat.");
    int base = keys.nextInt();

    System.out.println("Enter the IV.");
    int iv = keys.nextInt();

    System.out.println("Enter the EV.");
    int ev = keys.nextInt();

    System.out.println("Enter the level.");
    int level = keys.nextInt();
    if(level<1 || level>100)
    {
      System.out.println("Error! Invalid level!");
      System.exit(0);
    }

    System.out.println("Is the nature increase, decrease, or neutral?");
    char nature;
    nature = keys.nextLine().toLowerCase().charAt(0);

    if(nature=='i')
      natureMult=1.1;
    else if(nature=='n')
      natureMult=1.0;
    else if(nature=='d')
      natureMult=0.9;
    else
    {
      System.out.println("Error! You entered an invalid multiplier!");
      System.exit(0);
    }

    System.out.println("Is the stat HP?");
    hp = keys.nextLine().toLowerCase().charAt(0);
    if(hp!='y' || hp!='n')
    {
      System.out.println("Error! You did not pick yes or no!");
      System.exit(0);
    }

    if(hp=='y')
      finalStat = hpCalc(base, iv, ev, level);
    else
      finalStat = statCalc(base, iv, ev, level, natureMult);
  }

  public void writeOutput()
  {
    if(hp=='y')
      System.out.println("The final stat is: " + finalStat);
    else if(hp=='n')
      System.out.println("The final stat is: " + finalStat);
  }

  private int hpCalc(int a, int b, int c, int d)
  {
    int stat;
    stat = (int)((b+2*a+c/4.0+100)*d/100+10);
    return stat;
  }

  private int statCalc(int e, int f, int g, int h, double i)
  {
    int stat;
    stat = (int)(((f+2*e+g/4.0)*g/100+5)*h);
    return stat;
  }
}