#include<stdio.h>
int main()
{
    int count;
    scanf("%d",&count);
    int sum=2;
    int fibo;
    int i;
    int first=1;
    int second=1;
    for(i=1; i<=count-2; i++)
    {
        fibo=first+second;
        sum=sum+fibo;
        first=second;
        second=sum;


    }

    printf("%d",sum);







    return 0;
}