Ciklas sudeda suma

#include <fstream>
#include <iostream>

using namespace std;

int main() {
  ifstream df ("U1.txt");
  ofstream rf ("U1rez.txt");
  int a,n;
  df >> a >> n;
  int suma=0;
  for (int i = a; i <= n; i++) 
  {
  suma=suma + i;
  
  }
  rf<<suma;
  df.close();
  rf.close();
  return 0;
}