在序列1、6、15、28、45，…中找到第N个数字。

给定一个序列的前两个数字。任务是找到该系列的第N个(N最多为10 ^ 18)个数字。

注意：数组中的每个元素均比其前后数的平均值小2。答案可能非常大，因此请以模数10 ^ 9 + 9打印答案。

例子：

Input: N = 3
Output: 15
(1 + 15)/2 - 2 = 6

Input: N = 4
Output: 28
(6 + 28)/2 - 2 = 15

观察：根据声明，形成的级数将是1、6、15、28、45…..因此，第N个项的公式为：

2*n*n - n

C++

// CPP program to find Nth term of the series
#include 
using namespace std;
#define mod 1000000009
  
// function to return nth term of the series
int NthTerm(long long n)
{
    long long x = (2 * n * n) % mod;
    return (x - n + mod) % mod;
}
  
// Driver code
int main()
{
    long long N = 4;
  
    // function call
    cout << NthTerm(N);
  
    return 0;
}

Java

// Java program to find N-th
// term of the series:
  
import java.util.*;
import java.lang.*;
import java.io.*;
  
class GFG {
  
    // function to return nth term of the series
    static long NthTerm(long n)
    {
        long x = (2 * n * n) % 1000000009;
        return (x - n + 1000000009) % 1000000009;
    }
  
    // Driver Code
    public static void main(String args[])
    {
  
        // Taking  n as 6
        long N = 4;
  
        // Printing the nth term
        System.out.println(NthTerm(N));
    }
}

Python

# Python 3 program to find  
# N-th term of the series:  
    
  
# Function for calculating  
# Nth term of series  
def NthTerm(N) :  
    
    # return nth term
    x = (2 * N*N)% 1000000009
    return ((x - N + 1000000009)% 1000000009)  
    
# Driver code  
if __name__ == "__main__" :  
        
    N = 4
    
    # Function Calling  
    print(NthTerm(N))

C#

// C# program to find N-th
// term of the series:
using System;
  
class GFG 
{
  
// function to return nth
// term of the series
static long NthTerm(long n)
{
    long x = (2 * n * n) % 1000000009;
    return (x - n + 1000000009) %
                    1000000009;
}
  
// Driver Code
public static void Main()
{
  
    // Taking n as 6
    long N = 4;
  
    // Printing the nth term
    Console.WriteLine(NthTerm(N));
}
}
  
// This code is contributed
// by inder_verma

输出：

28