#include<iostream> // cin\cout\endl #include<cstdio> //scanf()\printf() #include<cstring> // strcpy()\strcat()\strcmp()\strlen()\memset() #include<cmath> //sqrt()\pow()\abs()\ceil()\floor()\max()\min() using namespace std; int main(){ int a[21]={1,1,2,3},b; cin>>b; for(int i=4;i<=b;i++){ a[i]=a[i-1]+a[i-2]; } cout<a[b]; return 0; }