import cs1.Keyboard;
public class SumFirst {
   public static int sumfirst(int n) {
     if (n<2) return n; else return n + sumfirst(n-1);
   }
   public static void main(String[] args) {
      System.out.println("This program sums the first n integers.");
      System.out.println("Enter the number n.");
	int n = Keyboard.readInt();
      if (n < 0)
         System.out.println("Error: n must be a positive integer.");
	else
         System.out.println("The sum of the first n integers is " + sumfirst(n));
   }
}