#include <iostream>
#include <cstdio>
#define LL long long int
using namespace std;

LL N;

int main(){
    freopen("nunta.in","r",stdin);
    freopen("nunta.out","w",stdout);
    scanf("%lld",&N);

    LL f=1,l=2,i,aux;
    for (i=3; i<=N; i++){
        aux=l;
        l=l+f;
        f=aux;
    }

    printf("%lld\n",l);
    return 0;
}