package main

import "fmt"

func main() {
    var n int
    fmt.Scan(&n)
    dp := make([]int64, n)
    dp[0] = 1
    dp[1] = 2
    for i := 2; i < n; i++ {
        dp[i] = dp[i-1] + dp[i-2]
    }
    fmt.Println(dp[len(dp)-1])
}