#include<stdio.h>
int n;
void solve(){
    long long unsigned i,a,b,aux;
    scanf("%d",&n);
    a=1;
    b=1;
    for(i=3; i<=n+1; ++i){
        aux=b;
        b+=a;
        a=aux;}
    printf("%lld",b);}
int main(){
    freopen("nunta.in","r",stdin);
    freopen("nunta.out","w",stdout);
    solve();
    return 0;}