Go: package main

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

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

func (s *Scanner) nextStr() string {
	s.sc.Scan()
	return s.sc.Text()
}

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N := sc.nextInt()
	dp := make([]int, N+1)
	dp[0] = 1
	dp[1] = 1
	for i := 2; i <= N; i++ {
		dp[i] = dp[i-1] + dp[i-2]
	}

	fmt.Fprintln(wtr, dp[N])
	wtr.Flush()
}

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;
}