import java.io.*;
public class TriangularNumber { // begin class
   public static void main(String[] s) throws IOException { // begin main
      BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));

      System.out.println("enter n number : " );
      int n = Integer.parseInt(stdIn.readLine());
      if (checkTriangular(n))
        System.out.println(n + " is a triangular number.");
      else
        System.out.println(n + " is not a triangular number.");
   } // end main
                               
   /* return true if the argument nth is triangular
      otherwise false
   */
   public static boolean checkTriangular(int nth){ // begin
     int sum = 0;
     boolean isTriangular = false;
     for (int i = 1; sum < nth; i++) { // begin for
       sum += i;
       if(sum == nth)
         isTriangular = true;  
     } // end for
     return isTriangular;
   } // end
} // end class

// Finds if a given number is a triangle number or not.
 
import java.io.*;
 
public class TriangularNumber {
        public static void main(String[] s) throws IOException {
 
                BufferedReader stdIn =
                        new BufferedReader(new InputStreamReader(System.in));
                System.out.println("enter a number : " );
                int n = Integer.parseInt(stdIn.readLine());
 
                if (checkTriangular(n))
                        System.out.println(n + " is a triangular number.");
                else
                System.out.println(n + " is not a triangular number.");
        }
 
        public static boolean checkTriangular(int num) {
                float r1 = 0, r2 = 0;
                float det = 0;
 
                det = (float)Math.sqrt(1 + 8 * num);
 
                if (det < 0) // Imaginary roots, => not Triangle #
                        return false;
 
                r1 = (det - 1) / 2;
                r2 = (det + 1) / 2;
 
                if (r1 > 0) {
                        if (r1 == (int)r1)
                                return true; // Triangle #
                }
 
                if (r2 > 0) {
                        if (r2 == (int)r2)
                                return true; // Triangle #
                }
 
                return false; //  not Triangle #
        }
}

                float d = 0, det = 0;
                det = (1+8*num);
 
                if (det < 0) // Imaginary roots, => not Triangle #
                        return false;
 
                d = (float)Math.sqrt(det);
 
                r1 = (d - 1) / 2;
                r2 = (d + 1) / 2;