#include<stdio.h>
int main()
{
    int count;
    scanf("%d",&count);
    int sum=0;
    int fibo;
    int i;
    for(i=1; i<=count; i++);
    {
        int first=1;
        int second=1;
        fibo=first+second;
        sum=sum+fibo;
        first=second;
        second=sum;

    }

    printf("%d",sum);






    return 0;
}