// 2021
#include<bits/stdc++.h>
using namespace std;

//constant define things

#define pb(a) push_back(a);
#define pf(a) push_front(a);

#define ff first
#define ss second

#define all(s) s.begin(),s.end()

#define pii pair<int,int>
#define pll pair<long,long>

bool isVowel(char a){
    a=tolower(a);
    return (a=='a'||a=='e'||a=='i'||a=='o'||a=='u'?true:false);
}
double find_dist(int x1,int y1,int x2,int y2){
    return sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
}
int to_int(string s){
    // stoi() ->function to int up to 9.2.0 versions
    int n=0,i=0;
    if(s[0]=='-')i=1;
    for(;i<(int)s.size();i++) n=n*10+(int(s[i])-48);
    if(s[0]=='-')n*=-1;
    return n;
}
string to_str(int X){
    // bug is that -9123 is not works because of sign '-'
    //to_string() -> function for c++ 17
    string ANS="";
    while(X>0){
        ANS+=(X%10)+'0';
        X/=10;
    }
    reverse(ANS.begin(),ANS.end());
    return ANS;
}
int BinaryToDecimal(int x){
    int cnt=0,ans=0;
    while(x>0){
        if(x%10==1){
            ans+=pow(2,cnt);
        }
        x/=10;
        cnt++;
    }
    return ans;
}
string decimalTObinary(int n) {
    string s="";
    if(n==0)return 0;
    else{
        for(int i=0;n>0;i++){
            s+=((n%2)+'0');
            n/=2;
        }
        return s;
    }
}
bool isPrime(int x){
    for(int i=2;i<=sqrt(x);i++){
        if(x%i==0){
            return false;
        }
    }
    return true;
}
int gcd(int a,int b){
    if(a==0) return b;
    return gcd(b%a,a);
}
int lcm(int a,int b){
    return a*(b/gcd(a,b));
}
vector<int>All_Factors(int n){
    vector<int>factors;
    for(int i=1;i<=sqrt(n);i++){
        if(n%i==0){
            factors.push_back(i);
            if(i!=n/i){
                factors.push_back(n/i);
            }
        }
    }
    return factors;
}
// %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

#define int long long
const int N=2e4,INF=1e9+7;
vector<int>dp(101,0);

int FFF(int x){
    if(x==1)return 2;
    if(x==2)return 3;
    if(x==3)return 5;

    if(dp[x])return dp[x];
    return dp[x]=FFF(x-1)+FFF(x-2);
}
void bbb(){
    int n;
    cin>>n;
    cout<<FFF(n);
}
signed main(){
    ios_base::sync_with_stdio(false); cin.tie(nullptr);cin.tie(nullptr);
    int test_cases=1;
    //cin>>test_cases;
    while(test_cases-->0){
        bbb();
    }
    return 0;
}