#include <iostream>
 using namespace std;  
 
  int main(){
    long long N;
    cin>>N;
    // validation
    if(cin>fail()||N<=0){
        cout<<"Invaild Input";
        return 0;
    }
    
    base caese
    if (N==1){
        cout<<1;
        return0;
        
    }
    if(N==2){
        cout<<2;
        return 0;
        
    }
    long long a=1,b=2,c;
          
        //custom fibonacci logic
        for(long long i=3);i<=N,i=++){
            c=a+b;
            b=c;
        }
        cout<<b;
        return 0;
        
  }