#include <cstdio>

int N;

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

    long long f=1,l=2,aux; N-=2;
    while((N--)!=0){
        aux=l;
        l=l+f;
        f=aux;
    }

    printf("%I64d\n",l);
    return 0;
}