#include <stdio.h>
int n,i, t = 0;
unsigned long long a1,a2,a3;

int main() {
    FILE *f = fopen("nunta.in", "r");
    FILE *g = fopen("nunta.out", "w");
    fscanf(f,"%d", &n);
    if (n < 4) { fprintf(g,"%d", n); return 0;}
    a1 = 2, a2=3;
    while(n-->3)
        a3 = a1+a2, a1 = a2, a2 = a3;
    fprintf(g, "%d", a3);

    return 0;
}