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 <stack>
#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
using namespace std;
int gcd(int a, int b)
{
   if (a%b == 0)
   {
       return(b);
   }
   else
   {
       return(gcd(b, a%b));
   }
}
int mod = 1000000000 + 7;
int lcs(string x, string y){
    
    int m = x.size();
    int n = y.size();
    int c[m+1][n+1];
    int Max = 0;
    string X = " " + x;
    string Y = " " + y;
    for(int j = 0; j <= m; j++){
        c[j][0] = 0;
    }
    for(int j = 1; j <= n; j++){
        c[0][j] = 0;
    }
    for(int i = 1; i <= m; i++){
        for(int j = 1; j <= n; j++){
            if(X[i] == Y[j]){
                c[i][j] = c[i-1][j-1] + 1;
            }else{
                c[i][j] = max(c[i-1][j], c[i][j-1]);
            }
            Max = max(Max, c[i][j]);
        }
    }
    
    return Max;
}
int main(){
    //std::stack<int> stack;
    
    int n;
    cin >> n;
    vector<int> fib(45);
    fib[0] = 1;
    fib[1] = 1;
    for(int i = 2; i < 45; i++){
        fib[i] = fib[i-1] + fib[i-2];
    }
    cout << fib[n] << endl;
}

C++: #include <iostream>

using namespace std;

int main() {

    int fib[50];
    fib[0] = fib[1] = 1;

    for (int i = 2; i <= 44; ++i) fib[i] = fib[i-1] + fib[i-2];

    int n;
    cin >> n;

    cout << fib[n] << endl;

    return 0;
}

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

int fib(int n);

vector<long long> fib_n(100, 0);

int main() {
  int n;
  cin >> n;
  cout << fib(n) << endl;
  return 0;
}

int fib(int n) {
  if (fib_n[0] == 0) {

    fib_n[0] = 1;
    fib_n[1] = 1;
  }
  if (n == 0) {
    return fib_n[0];
  }
  if (n == 1) {
    return fib_n[1];
  }
  if (fib_n[n] == 0) {
    fib_n[n] = fib(n - 1) + fib(n - 2);
  }
  return fib_n[n];
}

C++: #include <iostream>
#include <cstdio>
using namespace std;
int fib(int n){
  if(n==0) return 1;
  if(n==1) return 1;
  return fib(n-1)+fib(n-2);
}

int main() {
  int n;
  scanf("%d",&n);
  printf("%d\n",fib(n));
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
//#include "libs.h"
#include <random>
#include <unordered_map>
#include <unordered_set>
//#include <opencv2/core.hpp>
//#include <opencv2/highgui.hpp>
//#include <opencv2/imgproc.hpp>

using namespace std;

//呪文
#define DUMPOUT cerr
#define dump(...) DUMPOUT<<"  ";DUMPOUT<<#__VA_ARGS__<<" :["<<__LINE__<<":"<<__FUNCTION__<<"]"<<endl;DUMPOUT<<"    ";dump_func(__VA_ARGS__)

typedef unsigned uint; typedef long long ll; typedef unsigned long long ull; typedef pair<int, int> pii; typedef pair<ll, ll> pll; typedef pair<double, double> pdd; typedef pair<string, string> pss;
template <typename _KTy, typename _Ty> ostream& operator << (ostream& o, const pair<_KTy, _Ty>& m) { o << "{" << m.first << ", " << m.second << "}"; return o; }
template <typename _KTy, typename _Ty> ostream& operator << (ostream& o, const map<_KTy, _Ty>& m) { if (m.empty()) { o << "{ }"; return o; } o << "{" << *m.begin(); for (auto itr = ++m.begin(); itr != m.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _KTy, typename _Ty> ostream& operator << (ostream& o, const unordered_map<_KTy, _Ty>& m) { if (m.empty()) { o << "{ }"; return o; } o << "{" << *m.begin(); for (auto itr = ++m.begin(); itr != m.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const vector<_Ty>& v) { if (v.empty()) { o << "{ }"; return o; } o << "{" << v.front(); for (auto itr = ++v.begin(); itr != v.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const set<_Ty>& s) { if (s.empty()) { o << "{ }"; return o; } o << "{" << *(s.begin()); for (auto itr = ++s.begin(); itr != s.end(); itr++) { o << ", " << *itr; } o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const unordered_set<_Ty>& s) { if (s.empty()) { o << "{ }"; return o; } o << "{" << *(s.begin()); for (auto itr = ++s.begin(); itr != s.end(); itr++) { o << ", " << *itr; }	o << "}"; return o; }
template <typename _Ty> ostream& operator << (ostream& o, const stack<_Ty>& s) { if (s.empty()) { o << "{ }"; return o; } stack<_Ty> t(s); o << "{" << t.top(); t.pop(); while (!t.empty()) { o << ", " << t.top(); t.pop(); } o << "}";	return o; }
template <typename _Ty> ostream& operator << (ostream& o, const list<_Ty>& l) { if (l.empty()) { o << "{ }"; return o; } o << "{" << l.front(); for (auto itr = ++l.begin(); itr != l.end(); ++itr) { o << ", " << *itr; } o << "}"; return o; }
template <typename _KTy, typename _Ty> istream& operator >> (istream& is, pair<_KTy, _Ty>& m) { is >> m.first >> m.second; return is; }
template <typename _Ty> istream& operator >> (istream& is, vector<_Ty>& v) { for (size_t i = 0; i < v.size(); i++) is >> v[i]; return is; }
namespace aux { // print tuple
	template<typename Ty, unsigned N, unsigned L> struct tp { static void print(ostream& os, const Ty& v) { os << get<N>(v) << ", "; tp<Ty, N + 1, L>::print(os, v); } };
	template<typename Ty, unsigned N> struct tp<Ty, N, N> { static void print(ostream& os, const Ty& v) { os << get<N>(v); } };
}
template<typename... Tys> ostream& operator<<(ostream& os, const tuple<Tys...>& t) { os << "{"; aux::tp<tuple<Tys...>, 0, sizeof...(Tys)-1>::print(os, t); os << "}"; return os; }

template<typename A, size_t N, typename T> inline void Fill(A(&array)[N], const T &val) { std::fill((T*)array, (T*)(array + N), val); }

void dump_func() { DUMPOUT << endl; }
template <class Head, class... Tail> void dump_func(Head&& head, Tail&&... tail) { DUMPOUT << head; if (sizeof...(Tail) == 0) { DUMPOUT << " "; } else { DUMPOUT << ", "; } dump_func(std::move(tail)...); }

#define PI 3.14159265358979323846
#define EPS 1e-10
#define FOR(is,a,n) for(int is=(a);is<(n);++is)
#define REP(is,n)  FOR(is,0,n)
#define all(j) (j).begin(), (j).end()
#define SZ(j) ((int)(j).size())
#define fake false



unordered_map<ll, ll> memo;
ll fib(ll n) {
	if (memo.find(n) != memo.end())
		return memo[n];

	if (n <= 1)
		return memo[n] = 1;
	else
		return memo[n] = fib(n - 1) + fib(n - 2);
}

int main() {
	
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll n;
	cin >> n;
	cout << fib(n) << endl;

	return 0;
}

C++: #include<cstdio>

int main(){
    int a[44],n;
    a[0]=a[1]=1;
    for(int i=2;i<=44;i++) a[i]=a[i-1]+a[i-2];
    scanf("%d",&n);
    printf("%d\n",a[n]);
    return 0;
}

C++: #include<stdio.h>
#include<stream>
using namespace std;

int fibo(int x) {
  int a;
  if ( x == 0 ) return 1;
  if ( x == 1 ) return 1;
  a = fibo(x-1) + fibo(x-2);
  return a;
}

int main() {
  int x, y;

  cin >> x;

  y = fibo(x);

  cout << y << endl;

  return 0;
}

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

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

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

/*
long long fib(long long n){
    if(memo[n] != -1) return memo[n];
    else if(n == 0 || n == 1) return memo[n] = 1;
    else return memo[n] = fib(n - 1) + fib(n - 2);
}
*/

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