C :

#include<stdio.h>
int main()
{
	int n,i,s=0,f=1;
	scanf("%d",&n);
	for(i=1;i<=n;i++)
	{
		s=s+i*f;
		f=-f;
	}
	printf("%d\n",s);
}

C++ :

#include<stdio.h>
int main()
{
    int i,n,sum=0,f=1;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
    	sum=sum+i*f;
    	f=-f;
	}
	printf("%d",sum);
    return 0;
}

Java :

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int sum =0;
        for (int i = 1 ; i <= n ; i++){
            sum+=i*Math.pow(-1,i+1);
        }
        System.out.println(sum);
    }
}

Python :

# coding=utf-8
#python2
n = int(input())
s = 0
for i in range(1,n+1):
    if i%2==0:
        i = -i
    s += i
print(s)

C# :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp2
{
    internal class Program
    {
        static void Main()
        {
            int num = 0;
            int n = Convert.ToInt32(Console.ReadLine());
            for (int i = 1; i <= n; i++)

            {
                if (i %2== 0)
                {
                    i = -i;
                    num += i;
                    i = -i;
                }
                else if (i %2== 1)
                {
                    num += i;
                }

                
            }
            Console.WriteLine(num); 
            Console.ReadKey();
     
        }
    }
}