#include<stdio.h>
main(){
    int N;
    long long dp[50];
    dp[0] = 1;
    dp[1] = 2;
    scanf("%d",&N);
    for(int i = 2;i < N;i++){
        dp[i] = dp[i-1] + dp[i-2];
    }
    printf("%lld",dp[N-1]);
}