package main

import "fmt"

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