#include <stdio.h>
int main()
{int N;
 long long b,i,c,x;
    freopen("nunta.in","r",stdin);
    freopen("nunta.out","w",stdout);
    scanf("%d",&N);
    b=1;c=2;
    for(i=3;i<=N;i++)
     x=b+c,b=c,c=x;
     printf("%lld",x);
     return 0;
     }