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#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
int dp[41][2];
int solve(int n, int e) {
	if (n == 1) return 1;
	if (dp[n][e] != -1)
		return dp[n][e];
	int res = solve(n - 1, 0);
	if (!e) res += solve(n - 1, 1);
	return dp[n][e] = res;
}
signed main() {
	int n;
	scanf("%d", &n);
	for (int i = 0; i < 41; ++i)
		dp[i][0] = -1, dp[i][1] = -1;
	printf("%d", solve(n, 0) + solve(n, 1));
}