
function Main(input) {
  var N = parseInt(input.trim(), 10);
  var vec_fX = Array.from({length: N}, () => 1);
  var limit;
  var ans = 0;
  for (var i = 1; i < N; i++){
    limit = N / (i + 1);
    for (var j = 1; j <= limit; j++){
      ans += (i + 1) * j;
    }
  }
  ans += (N * (N + 1)) / 2;
  console.log(ans);
}
 
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
