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<iostream>
#define MAX_N 45
int F[MAX_N];

int fib(int n){
   if(F[n]>0) return F[n];
   if(n==0 || n==1) return F[n]=1;
   return F[n]=F[n-1]+F[n-2];
}

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

C++: #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 44
int F[MAX]={};
int fibonacci(int);

int main()
{
  int n,memo;

  scanf("%d",&n);

  printf("%d\n",fibonacci(n));

  return 0;
}

int fibonacci(int n){
  if(n==0||n==1) return F[n]=1;
  if(F[n]!=0) return F[n];
  return F[n]=fibonacci(n-2)+fibonacci(n-1);
}

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

int main(){
    int n;cin>>n;
    int F[n+1]={};F[0]=1;F[1]=1;
    for(int i=2;i<n+1;i++){
        F[i]=F[i-1]+F[i-2];//小さい値から計算していくので無駄がない.
    }
    cout<<F[n]<<endl;
    return 0;
}

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

#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT_ON(bit, i) (((bit >> i) & 1) > 0)

typedef long long LL;
template<typename T> std::vector<T> make_v(size_t a){return std::vector<T>(a);}
template<typename T,typename... Ts> auto make_v(size_t a, Ts... ts){ return std::vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));}    // C++14
template<typename T,typename V> typename std::enable_if<std::is_class<T>::value==0>::type fill_v(T &t,const V &v){t=v;}
template<typename T,typename V> typename std::enable_if<std::is_class<T>::value!=0>::type fill_v(T &t,const V &v){for(auto &e:t) fill_v(e,v);}
template<class T> inline T ceil(T a, T b) { return (a + b - 1) / b; }
template<class T> inline void print(T x) { std::cout << x << std::endl; }
template<class T> inline void print_vec(const std::vector<T> &v) { for (int i = 0; i < v.size(); ++i) {  if (i != 0) {std::cout << " ";} std::cout << v[i];} std::cout << "\n"; }
template<class T> inline bool inside(T y, T x, T H, T W) {return 0 <= y and y < H and 0 <= x and x < W; }
template<class T> inline double euclidean_distance(T y1, T x1, T y2, T x2) { return sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)); }
template<class T> inline double manhattan_distance(T y1, T x1, T y2, T x2) { return abs(x1 - x2) + abs(y1 - y2); }
template<typename T> T &chmin(T &a, const T &b) { return a = std::min(a, b); }
template<typename T> T &chmax(T &a, const T &b) { return a = std::max(a, b); }

template<class T> inline std::vector<T> unique(std::vector<T> v) {
    sort(v.begin(), v.end());
    v.erase(unique(v.begin(), v.end()), v.end());
    return v;
}
// 初項s, 交差dのn個の数列の和
long long sum_of_arithmetic_progression(long long s, long long d, long long n) {
    return n * (2 * s + (n - 1) * d) / 2;
}

// 2のべき乗数かどうか
bool is_power_of_two(long long x) {
    return !(x & (x - 1));
}

// aとbの最大公約数 O(log min(a, b))
long long gcd(long long a, long long b) {
    if (b == 0) { return a; }
    return gcd(b, a % b);
}

// 数列vの最大公約数 O(N log d)
long long gcd(std::vector<long long> &v) {
    long long ans = v[0];
    for (int i = 1; i < v.size(); ++i) {
        ans = gcd(ans, v[i]);
    }
    return ans;
}

// aとbの最小公倍数 O(log min(a, b))
long long lcm(long long a, long long b) {
    long long g = gcd(a, b);
    return a / g * b;
}

const int INF = 1 << 30;
const double EPS = 1e-9;
const long double PI = acos(-1.0);
const std::vector<int> dy2 = {0, 1}, dx2 = {1, 0};  // 右，下
const std::vector<int> dy4 = {0, -1, 0, 1}, dx4 = {1, 0, -1, 0};    // 右，上，左，下
const std::vector<int> dy8 = { 0, -1, 0, 1, 1, -1, -1, 1 }, dx8 = { 1, 0, -1, 0, 1, 1, -1, -1 };


using namespace std;


class Vector {
public:
    int n;
    vector<long long> v;
    Vector(int n) : n(n) {
        v.resize(n);
    }
};

class Matrix {
public:
    int m;
    int n;
    vector<vector<long long>> matrix;

    // m行n列の行列
    Matrix(int m, int n) : m(m), n(n) {
        matrix.resize(m, vector<long long>(n));
    }

    // m行m列の単位行列を生成
    static Matrix identity_matrix(int m) {
        auto a = Matrix(m, m);
        for (int i = 0; i < m; ++i) {
            a.matrix[i][i] = 1;
        }
        return a;
    }

    void multiple(const Matrix &a, const long long mod = 0LL) {
        assert(this->n == a.m);
        Matrix b = Matrix::matrix_multiple(*this, a, mod);
        this->m = b.m;
        this->n = b.n;
        this->matrix = b.matrix;
    }

    static Matrix matrix_multiple(const Matrix &a, const Matrix &b, const long long mod = 0LL) {
        assert(a.n == b.m);
        Matrix c(a.m, b.n);
        for (int i = 0; i < a.m; ++i) {
            for (int k = 0; k < a.n; ++k) {
                for (int j = 0; j < b.n; ++j) {
                    if (mod == 0) {
                        c.matrix[i][j] = c.matrix[i][j] + a.matrix[i][k] * b.matrix[k][j];
                    }
                    else {
                        c.matrix[i][j] = c.matrix[i][j] + a.matrix[i][k] * b.matrix[k][j];
                        c.matrix[i][j] %= mod;
                    }
                }
            }
        }

        return c;
    }

    void pow(const long long k, const long long mod = 0LL) {
        assert(this->m == this->n);
        auto a = Matrix::matrix_pow(*this, k, mod);
        this->m = a.m;
        this->n = a.n;
        this->matrix = a.matrix;
    }

    // a^k
    static Matrix matrix_pow(Matrix a, long long k, const long long mod = 0LL) {
        assert(a.m == a.n);
        const int n = a.m;

        Matrix b = Matrix::identity_matrix(n);
        while (k > 0) {
            if (k % 2 == 1) {
                b = Matrix::matrix_multiple(b, a, mod);
            }
            a = Matrix::matrix_multiple(a, a, mod);
            k /= 2;
        }
        return b;
    }

    void show() {
        for (int i = 0; i < this->m; ++i) {
            for (int j = 0; j < this->n; ++j) {
                cout << this->matrix[i][j] << " ";
            }
            cout << endl;
        }
    }
};

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

    Matrix a(2, 2);
    a.matrix[0][0] = 1;
    a.matrix[0][1] = 1;
    a.matrix[1][0] = 1;
    a.matrix[1][1] = 0;

    a.pow(N + 1);

    print(a.matrix[1][0]);

    return 0;
}

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

int fib(int n){
  int a = 1, b = 1, tmp;
  for(int i = 1; i < n; i++){
    tmp = a;
    a = b;
    b = tmp+b;
  }

  return b;
}

int main(){
  int n, ans;
  cin >> n;

  ans = fib(n);
  cout << ans << endl;

  return 0;
}