#include <fstream>
using namespace std;
FILE *f =fopen ("nunta.in", "r");
FILE *g =fopen ("nunta.out", "w");
unsigned char a[210]={'0'}, b[210]={'0'};
short n, A, B, z, aux, c, i, j;
int main()
{
    fscanf(f, "%hd", &n);
    if (n==1)
    {
        fprintf(g, "1");

        return 0;
    }
    if (n==2)
    {
        fprintf(g, "2");


        return 0;
    }
    if (n==3)
    {
        fprintf(g, "3");

        return 0;
    }
    a[0]=2;
    b[0]=3;
    for (j=4; j<=n; j++)
    {
        z=0;
        i=0;
        while (i<=A||i<=B||z!=0)
        {

            aux=a[i];
            a[i]=b[i];
            c=aux+a[i]+z;
            b[i]=c%10;
            z=c/10;
            i++;
        }
        A=B;
        B=i-1;
    }
    for (i=B; i>=0; i--)
        fprintf(g, "%c", b[i] + '0');
    return 0;
}