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++: //
//  main.cpp
//  ALDS1_10_A:FibonacciNumber
//
//  Created by Kang on 2020/8/12.
//  Copyright © 2020 Kang. All rights reserved.
//

#include <cstdio>
#include <cstring>
using namespace std;


int fiboFromTop(int n, int fibo[]) {
    if (fibo[n] != 0) {
        return fibo[n];
    }
    
    if (n == 0 || n == 1) {
        fibo[n] = 1;
        return 1;
    }
    else {
        fibo[n - 1] = fiboFromTop(n-1, fibo);
        fibo[n - 2] = fiboFromTop(n - 2, fibo);
        return fibo[n - 1] + fibo[n - 2];
    }
    
    
    return 0;
}

int fiboFromBottom(int n) {
    if (n == 0 || n == 1) {
        return 1;
    }
    int fibo[45];
    fibo[0] = 1;
    fibo[1] = 1;
    for (int i = 2; i <= n; i++) {
        fibo[i] = fibo[i-1] + fibo[i - 2];
    }
    return fibo[n];
}

int main(int argc, const char * argv[]) {
    // insert code here...
    int n;
    int fibonacci[45];
    memset(fibonacci, 0, 45 * sizeof(int));
    scanf("%d", &n);
    
    // printf("%d\n", fiboFromTop(n, fibonacci));
    printf("%d\n", fiboFromBottom(n));
    
    return 0;
}

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

int main(){
  int n,for_n=1,pre_n=1;

  cin >> n;
  for(int i=0;i<=n;i++){
    if((i==0) || (i==1)){
      continue;
    }else{
      int swp=for_n;

      for_n += pre_n;
      pre_n = swp;
    }
  }
  cout << for_n << endl;
}

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

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

C++: #include<iostream>

using namespace std;

int fib(int n);

int main() {
  int n;

  //入力
  cin >> n;

  //出力
  cout << fib(n) << endl;

  return 0;
}

int fib(int n){
  int memo[1000];

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

C++: #include <iostream>
#include <algorithm>
#include <cstdio>
#include <limits>

const int N = 102;

long m[N][N], p[N+1];

int matrixChainOrder(int n){

  for(int i=1; i<n; i++){ m[i][i] = 0; }

  for(int l=2; l<n; l++){
    for(int i=1; i < n - l + 1; i++){
      int j = i+l-1;
      m[i][j] = std::numeric_limits<long>::max();
      for(int k=i; k<=j-1; k++){
        m[i][j] = std::min(m[i][j], m[i][k] + m[k+1][j] + p[i-1]*p[k]*p[j]);
      }
    }
  }
  return m[1][n - 1];
}

int main(){
  int n ;
  std::cin >> n; //number of matrix

  for(int i=1; i<=n; i++){
    std::cin >> p[i-1] >> p[i];
  }
  // for (int i = 0; i <=n; ++i) std::cout << p[i] << std::endl;
  std::cout << matrixChainOrder(n + 1) << std::endl;

  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for(int i=(int)(s-1); i>=(int)(e); i--)
#define pb push_back
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()
#define fi first
#define se second

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll INF = 1e18;
const ll MOD = 1e9 + 7;
const double EPS = 1e-8;

template<typename T> T chmax(T& a, const T& b){return a = (a > b ? a : b);}
template<typename T> T chmin(T& a, const T& b){return a = (a < b ? a : b);}

#define DEBUG_MODE
#ifdef DEBUG_MODE
#define dump(x) cout << #x << " : " << x << " "
#define dumpL(x) cout << #x << " : " << x << '\n'
#define LINE cout << "line : " << __LINE__ << " "
#define LINEL cout << "line : " << __LINE__ << '\n'
#define dumpV(v) cout << #v << " : ["; for(auto& t : v) cout << t << ", "; cout<<"]" << " "
#define dumpVL(v) cout << #v << " : ["; for(auto& t : v) cout << t << ", "; cout<<"]" << endl
#define STOP assert(false)
#else
#define dump(x) 
#define dumpL(x) 
#define LINE 
#define LINEL 
#define dumpV(v) 
#define dumpVL(v) 
#define STOP assert(false)
#endif
#define mp make_pair
 
namespace std {
template<class S, class T>
ostream &operator <<(ostream& out, const pair<S, T>& a) {
    out << '(' << a.fi << ", " << a.se << ')';
    return out;
}
}

template <class T, int SZ>
struct Mat {
    array<array<T, SZ>, SZ> d;
    const int n;
    Mat() : n(SZ) {
        array<T, SZ> tmp;
        d.fill(tmp);
    }
    Mat operator * (const Mat& mt) const {
        Mat ret;
        rep(i, SZ) rep(j, SZ) {
            rep(k, SZ) {
                ret.d[i][j] += (d[i][k] * mt.d[k][j]);
            }
        }
        return ret;
    }
    Mat& operator *= (const Mat& mt) {
        *this = *this * mt;
        return *this;
    }
    Mat& operator = (const Mat& mt) {
        d = mt.d;
        return *this;
    }
    array<T, SZ>& operator [](const int n) {
        return d[n];
    }
};

template <class T, int SZ> 
Mat<T, SZ> pow(Mat<T, SZ> mt, ll n) {
    Mat<T, SZ> ret;
    rep(i, SZ) ret.d[i][i] = 1LL;
    while(n > 0) {
        if(n&1LL) ret = ret * mt;
        mt = mt*mt;
        n >>= 1;
    }
    return ret;
}

template <typename T, typename U> T pow_fast(T x, U n) {
    T ret = (T)1;
    while(n) {
        if(n & (U)1) ret *= x;
        x *= x;
        n >>= 1;
    }
    return ret;
}

template <typename ModType, ModType MOD> struct ModInt {
    ModType val;
    ModInt() : val(0) {}
    ModInt(ModType x) : val(x % MOD) {
        if(val < 0) val += MOD;
    };
    operator ModType() const { return val; }
    ModInt &operator+=(const ModInt &m) {
        val += m.val;
        if(val >= MOD) val -= MOD;
        return *this;
    }
    ModInt &operator-=(const ModInt &m) {
        val -= m.val;
        if(val < 0) val += MOD;
        return *this;
    }
    ModInt &operator*=(const ModInt &m) {
        (val *= m.val) %= MOD;
        return *this;
    }
    ModInt &operator/=(const ModInt &m) {
        *this *= m.inverse();
        return *this;
    }
    ModInt operator+(const ModInt &m) const { return ModInt(*this) += m; }
    ModInt operator-(const ModInt &m) const { return ModInt(*this) -= m; }
    ModInt operator*(const ModInt &m) const { return ModInt(*this) *= m; }
    ModInt operator/(const ModInt &m) const { return ModInt(*this) /= m; }
    ModInt inverse() const { return pow_fast(*this, MOD - 2); }
    ModInt pow(ModType n) const {return pow_fast(*this, n); }

    friend std::ostream &operator<<(std::ostream &os, const ModInt &rhs) {
        os << rhs.val;
        return os;
    }
    friend std::istream &operator>>(std::istream &is, ModInt &rhs) {
        ModType value;
        is >> value;
        rhs = ModInt(value);
        return is;
    }
};

int main(){
    // using mint = ModInt<ll, MOD>;
    Mat<ll, 2> mt;
    mt[0][0] = mt[0][1] = mt[1][0] = 1;
    int n;
    cin >> n;
    cout << pow(mt, n+1)[1][0] << endl;
    return 0;
}

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

int main() {

    int n;
    cin >> n;

    int Fib[50] = {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;
    return 0;
}

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

int S[50];
int fib(int N){
    if(N==0||N==1) return S[N]=1;
    if(S[N]!=0) return S[N];
    return S[N]=fib(N-1)+fib(N-2);
}

int main(){

    int N;
    cin>>N;
    cout<<fib(N)<<endl;
}