Посчитать сумму целых чисел в заданном диапазоне

(1 + N)*N/2;

N/2*(1 + N);

N/2.0*(1 + N);

#include <iostream> 
#include <cmath>
using namespace std;
 
int main(void)
{
    freopen("input.txt","r", stdin);
    freopen("output.txt", "w", stdout);
    int n, s;
    cin >> n;   
    s = (1 + n) * (abs(n / 2.) + (n < 1));
    cout << s;
    return 0;
}

#include <iostream>
 
template <unsigned N>
struct sum {
   static const unsigned value = N + sum<N-1>::value;
};
 
template <>
struct sum<1> {
   static const unsigned value = 1;
};
 
int main() {
   const unsigned N = 10;
   std::cout << sum<N>::value << std::endl;
}

#include <fstream>
 
main()
{
    int n =
    #include "input.txt"
    ;
    std:: ofstream("output.txt") << (1 + abs(n)) * n / 2  + (n < 1);
}

#include <fstream>
 
template <unsigned N>
struct sum {
  static const unsigned value = N + sum<N-1>::value;
};
 
template <>
struct sum<1> {
  static const unsigned value = 1;
};
 
int main() {
  const unsigned N = 
    #include "input.txt"
  ;
  std:: ofstream("output.txt")  << sum<N>::value;
}