fn main() {
    proconio::input! {
        n: u64,
    }
    let mut dp = vec![0u64; 50];
    dp[0] = 1;
    dp[1] = 2;
    for i in 2..50 {
        dp[i] = dp[i - 1] + dp[i - 2];
    }
    println!("{}", dp[n as usize - 1]);
}