#include <cstdio>

using namespace std;

long long dp[55];

int main(void) {
  int n;
  scanf("%d", &n);
  dp[0] = 1;
  for (int i = 0; i <= n; i++) {
    dp[i+1] += dp[i];
    dp[i+2] += dp[i];
  }
  printf("%lld\n", dp[n]);
}