#include<cstdio>
#include<algorithm>
using namespace std;



short int  n,i,j,a[500],b[500],c[500];


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

    if  (n<3){
        if (n==1)printf("1");
         else printf("2");
        return 0;
    }

    a[0]=a[1]=1;
    b[0]=1;b[1]=2;
    for (i=3;i<=n;i++)
    {
        c[0]=max(a[0],b[0]);
    int t=0,j;
    for (j=1;j<=c[0];j++)
    {
        c[j]=a[j]+b[j]+t;
        t=c[j]/10;
        c[j]%=10;
    }
    if (t)c[++c[0]]=t;
        for (j=0;j<=b[0];j++)a[j]=b[j];
        for (j=0;j<=c[0];j++)b[j]=c[j];
    }
    for (i=c[0];i;i--)printf("%d",c[i]);
    return  0;
}