Encuentra la suma de la serie 1-2+3-4+5-6+7…….

Dado un número N. La tarea es encontrar la suma de la siguiente serie hasta el término ⁿ .
 

1- 2 + 3 – 4 + 5 – 6 +….

Ejemplos : 
 

Input : N = 8
Output : -4

Input : N = 10001
Output : 5001

Enfoque: Si observamos cuidadosamente, podemos ver que la suma de la serie anterior sigue un patrón de números enteros positivos y negativos alternos que comienzan de 1 a N, como se muestra a continuación:
 

N   = 1, 2, 3, 4, 5, 6, 7 ......
Sum = 1, -1, 2, -2, 3, -3, 4 ......

Por lo tanto, del patrón anterior, podemos concluir que: 
 

• cuando n es impar => suma = (n+1)/2
• cuando n es par => suma = (-1)*n/2

A continuación se muestra la implementación del enfoque anterior:
 

// C++ program to find the sum of
// series 1 - 2 + 3 - 4 +......
 
#include <iostream>
using namespace std;
 
// Function to calculate sum
int solve_sum(int n)
{
    // when n is odd
    if (n % 2 == 1)
        return (n + 1) / 2;
 
    // when n is not odd
    return -n / 2;
}
 
// Driver code
int main()
{
    int n = 8;
     
    cout << solve_sum(n);
     
    return 0;
}

// Java program to find sum of
// first n terms of the given series
import java.util.*;
 
class GFG
{
static int calculateSum(int n)
{
    // when n is odd
    if (n % 2 == 1)
        return (n + 1) / 2;
 
    // when n is not odd
    return -n / 2;
}
 
// Driver code
public static void main(String ar[])
{
 
// no. of terms to find the sum
int n = 8;
System.out.println(calculateSum(n));
}
}

# Python program to find the sum of
# series 1 - 2 + 3 - 4 +......
 
# Function to calculate sum
def solve_sum(n):
    # when n is odd
    if(n % 2 == 1):
        return (n + 1)/2
 
    # when n is not odd
    return -n / 2
 
# Driver code
n = 8
 
print(int(solve_sum(n)))

// C# program to find sum of
// first n terms of the given series
using System;
 
class GFG
{
static int calculateSum(int n)
{
    // when n is odd
    if (n % 2 == 1)
        return (n + 1) / 2;
 
    // when n is not odd
    return -n / 2;
}
 
// Driver code
public static void Main()
{
 
    // no. of terms to find the sum
    int n = 8;
    Console.WriteLine(calculateSum(n));
}
}
 
// This code is contributed
// by inder_verma

<?php
// PHP program to find the sum of
// series 1 - 2 + 3 - 4 +......
 
// Function to calculate sum
function solve_sum($n)
{
    // when n is odd
    if ($n % 2 == 1)
        return ($n + 1) / 2;
 
    // when n is not odd
    return -$n / 2;
}
 
// Driver code
$n = 8;
 
echo solve_sum($n);
 
// This code is contributed
// by inder_verma
?>

<script>
 
// javascript program to find sum of
// first n terms of the given series
 
 
 
function calculateSum(n)
{
    // when n is odd
    if (n % 2 == 1)
        return (n + 1) / 2;
 
    // when n is not odd
    return -n / 2;
}
 
// Driver code
 
// no. of terms to find the sum
var n = 8;
document.write(calculateSum(n));
 
 
// This code contributed by shikhasingrajput
 
</script>

-4