Go: package main

// import (
// 	"fmt"
// 	"bufio"
// 	"strconv"
// 	"os"
// )

// func fib(n int) int{
// 	if n == 0 || n == 1{
// 		return 1
// 	} else {
// 		return fib(n-1) + fib(n-2)
// 	}
// }

// func main(){
// 	sc := bufio.NewScanner(os.Stdin)
// 	sc.Scan()
// 	n, _ := strconv.Atoi(sc.Text())
// 	fmt.Println(fib(n))
// }

import (
	"fmt"
)

func main(){
	var n, pre1, pre2, now int
	fmt.Scan(&n)
	if n == 0 || n == 1{
		fmt.Println(1)
	} else {
		pre1, pre2 = 1, 1
		for i:=1; i<n; i++{
			now = pre1 + pre2
			pre2 = pre1
			pre1 = now
		}
		fmt.Println(now)
	}

}

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

#define rep(i, n) for (int i = 0; i < n; ++i)
#define ALL(a) (a).begin(), (a).end()

template <class T>
inline bool chmin(T &a, T b){
  if (a > b)
  {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T &a, T b){
  if (a < b)
  {
    a = b;
    return true;
  }
  return false;
}

using ll = long long;
using ld = long double;
using P = pair<int, int>;
using DoP = pair<double, double>;
using edge = struct { int to; ll cost; }; // edge x = {to, cost}

const int di[] = {0, 1, 0, -1, 1, 1, -1, -1};
const int dj[] = {1, 0, -1, 0, 1, -1, 1, -1};
const int INF = 400000000;
const ll INF64 = 1ll << 40;
const int mod = 1000000007;

struct Sieve
{
  int n;
  vector<int> f, primes;
  Sieve(int n = 1) : n(n), f(n + 1)
  {
    f[0] = f[1] = -1;
    for (ll i = 2; i <= n; ++i)
    {
      if (f[i])
        continue;
      primes.push_back(i);
      f[i] = i;
      for (ll j = i * i; j <= n; j += i)
      {
        if (!f[j])
          f[j] = i;
      }
    }
  }
  bool isPrime(int x) { return f[x] == x; }
  vector<int> factorList(int x)
  {
    vector<int> res;
    while (x != 1)
    {
      res.push_back(f[x]);
      x /= f[x];
    }
    return res;
  }
  vector<P> factor(int x)
  {
    vector<int> fl = factorList(x);
    if (fl.size() == 0)
      return {};
    vector<P> res(1, P(fl[0], 0));
    for (int p : fl)
    {
      if (res.back().first == p)
      {
        res.back().second++;
      }
      else
      {
        res.emplace_back(p, 1);
      }
    }
    return res;
  }
};
struct UnionFind
{
  vector<int> par;
  vector<int> siz;
  UnionFind(int N) : par(N), siz(N, 1) {for (int i = 0; i < N; i++) par[i] = i;}
  
  void init(int N) {
    siz.assign(N, 1);
    par.resize(N);
    for(int i = 0; i < N; ++i) par[i] = i;
  }

  int root(int x) {
    while(par[x] != x) {x = par[x] = par[par[x]];}
    return x;
  }

  bool unite(int x, int y)
  {
    x = root(x);
    y = root(y);
    if(x == y) return false;

    if(siz[x] < siz[y]) swap(x, y);
    siz[x] += siz[y];
    par[y] = x;
    return true;

  }

  bool isSame(int x, int y) {return root(x) == root(y); }

  int size(int x) { return siz[root(x)]; }
};
struct mint
{
  ll x; // typedef long long ll;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a)
  {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a)
  {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a)
  {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const
  {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const
  {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const
  {
    mint res(*this);
    return res *= a;
  }
  mint pow(ll t) const
  {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const
  {
    return pow(mod - 2);
  }
  mint &operator/=(const mint a)
  {
    return (*this) *= a.inv();
  }
  mint operator/(const mint a) const
  {
    mint res(*this);
    return res /= a;
  }
};
// h(n, r) = c(n+r-1, r)
struct combination
{
  vector<mint> fact, ifact;
  combination(int n) : fact(n + 1), ifact(n + 1)
  {
    assert(n < mod);
    fact[0] = 1;
    for (int i = 1; i <= n; ++i)
      fact[i] = fact[i - 1] * i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i)
      ifact[i - 1] = ifact[i] * i;
  }
  mint operator()(int n, int k)
  {
    if (k < 0 || k > n)
      return 0;
    return fact[n] * ifact[k] * ifact[n - k];
  }
};


int main(){
  int n;
  cin >> n;
  vector<ll> fib(n+1, 0);
  fib[0] = fib[1] = 1;
  for(int i = 2; i <= n; ++i) fib[i] = fib[i-1] + fib[i-2];
  cout << fib[n] << endl;
}

C++: #include <bits/stdc++.h>

using namespace std;

/*-------------------------------*/
#define FOR(i, a, b) for(int i = (a); i < (b); i++)  //a~(b - 1)?????§
#define all(i) (i).begin(),(i).end() //??¨??????(sort??¨???)
#define mp make_pair //mp(OO, OO)??¨???:::pint??¨???

typedef long long ll;
typedef pair<int, int> pint;
typedef vector<int> vi;
typedef vector<vector<int> > vii;
typedef vector<long long> llv;
typedef vector<pint> vpint;
/*-------------------------------*/

int main(void)
{
  int n; cin >> n;
  int dp[45] = {};
  dp[0] = 1; dp[1] = 1;

  FOR(i, 2, 45){
    dp[i] = dp[i - 1] + dp[i - 2];
  }
  cout << dp[n] << endl;
  return 0;
}

C++: #include <iostream>

int fibs[50] = {1, 1};

int fib(int n)
{
	if (n == 0 || n == 1) return fibs[n];
	if (fibs[n]) return fibs[n];
	return fibs[n] = fib(n - 1) + fib(n - 2);    // pay attension to here.
}

int main()
{
	int n;
	std::cin >> n;
	std::cout << fib(n) << std::endl;

	return 0;
}

C++: #include <iostream>

using namespace std;

int fib(int n);
int fib_rec(int n, int dp[]);

int main()
{
	int n;
	cin >> n;
	cout << fib(n) << endl;
}


int fib(int n)
{
	//亦可采用静态数组 
	int * dp = new int[n+1]();
	
	int result = fib_rec(n, dp);	
	
	delete [] dp;
	
	return result;	
	
}


int fib_rec(int n, int dp[])
{

	if (n == 1 || n == 0)
		return dp[n] = 1;
	if (dp[n] != 0)
		return dp[n];
	return dp[n] = fib_rec(n-1, dp) + fib_rec(n-2, dp);
}

C++: #include <bits/stdc++.h>

int main(void) {

  int Fi[64];
  int n;

  std::cin >> n;
  
  Fi[0] = 1;
  Fi[1] = 1;
  for(int i = 2; i <= n; ++i) {
    Fi[i] = Fi[i - 1] + Fi[i - 2];
  }

  std::cout << Fi[n] << std::endl;

  return 0;
}

C++: // lcmとかはg++ -std=c++17 default.cppみたいなかんじで
//vector<vector<ll>> piyo(tate,vector<ll> (yoko));

#include <bits/stdc++.h>
#define mod 1000000007
#define INF 1000000000
#define ll long long
#define ln cout<<endl;
#define Yes cout<<"Yes"<<endl
#define No cout<<"No"<<endl
#define double long double
#define REP(i,m,n) for(ll i=(ll)(m);i<(ll)(n);i++)
#define rep(i,n) REP(i,0,n)
using namespace std;
ll target;
bool my_compare(pair<string, ll> a, pair<string , ll> b) {
    // 基本はfirstで比較
    if(a.first != b.first){
         return a.first < b.first; // 昇順
        //return a.first > b.first; // 降順
    }

    // それ以外はsecondで比較
    if(a.second != b.second){
        return a.second > b.second;
    }else{
        // どちらも同じ
        return true;
    }
}

int main() {
    cin.tie(0);
   	ios::sync_with_stdio(false);
    //double mini=INF;
    ll maxi=0,f=0,sum=0,mini=INF;
    string str,str1,str2;
    ll i;
    double h;
    ll n,x,m;
    ll a,b,d,c;
    cin>>n;
    ll dp[50];
    dp[0]=1;
    dp[1]=1;
    for(i=2;i<46;i++){
        dp[i]=dp[i-1]+dp[i-2];
    }
    cout<<dp[n]<<endl;

    return 0;
}

C++: #include <iostream>
using namespace std;

int main(){
  int n;
  cin >> n;
  int F[50];
  F[0] = F[1] = 1;
  for(int i = 2; i <= n;i++) F[i] = F[i-1] + F[i-2];
  cout << F[n] << endl;
  return 0;
}

C++: #include <iostream>

using namespace std;

unsigned long long dp[50] = {0};

unsigned long long fib(int n){
  if(n == 0){
    return 1;
  } else if(n == 1){
    return 1;
  } else if(dp[n] != 0){
    return dp[n];
  } else {
    dp[n] = fib(n-1) + fib(n-2);
    return dp[n];
  }

}

int main(){
  int n;

  cin >> n;

  cout << fib(n) << endl;

  return 0;
}