#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int dp[101] = {0};
  dp[1] = 1;
  dp[2] = 2;
  dp[3] = 4;
  dp[4] = 6;
  for (int i = 5; i <= 100; i++) {
    dp[i] = 1;
    dp[i] += (i - 1);
    dp[i] += dp[i - 2];
  }
  cout << dp[n] << endl;
  return 0;
}
