21d)met1

#include <iostream>

using namespace std;

int main()
{

 unsigned long n,E=0,i=1;

 cout<<"n=";cin>>n;

 while (i<=n)
 {
     if (i%2==0)
        E-=2*i;
     else
        E+=2*i;
     i++;
 }
cout<<"E="<<E;

return 0;}