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 rep(i,n) for (int i = (0); i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define show(x) cout << #x << " = " << (x) << endl
#define show2(x,y) cout << #x << " = " << (x) << ", " << #y << " = " << (y) << endl
#define show3(x,y,z) cout << #x << " = " << (x) << ", " << #y << " = " << (y) <<  ", " << #z << " = " << (z) << endl
#define showv(v) rep(i,v.size()) printf("%d%c", v[i], i==v.size()-1?'\n':' ')
#define showv2(v) rep(j,v.size()) showv(v[j])
#define showt(t,n) rep(i,n) printf("%d%c", t[i], i==n-1?'\n':' ')
#define showt2(t,r,c) rep(j,r) showt(t[j],c)
#define showvp(p) rep(i,p.size()) printf("%d %d\n", p[i].first, p[i].second)
#define printv(v) rep(i,v.size()) printf("%d\n", v[i])
#define printt(t,n) rep(i,n) printf("%d\n", t[i])
#define incl(v,x) (find(all(v),x)!=v.end())
#define incls(s,c) (s.find(c)!=string::npos)
#define lb(a,x) distance((a).begin(),lower_bound(all(a),(x)))
#define ub(a,x) distance((a).begin(),upper_bound(all(a),(x)))
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define bit(n,k) ((n>>k)&1) // nのk bit目
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define uni(x) x.erase(unique(all(x)),x.end())
#define SP << " " <<
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vs = vector<string>;
using P = pair<int, int>;
using T = tuple<int, int, int>;
using vp = vector<P>;
using vt = vector<T>;
const int mod = 1000000007;
const double EPS = 1e-9;
//const long double EPS = 1e-14;
const int INF = (1<<30)-1;
const ll LINF = (1LL<<62)-1;
#define dame { puts("No"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
inline int in() { int x; cin >> x; return x;}
inline ll lin() { ll x; cin >> x; return x;}
inline char chin() { char x; cin >> x; return x;}
inline string stin() { string x; cin >> x; return x;}
inline double din() { double x; cin >> x; return x;}
//template<class T = int> inline T in() { T x; cin >> x; return (x);}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
char itoa(int n) { return n + '0';}
ll gcd(ll a, ll b) { return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b) { return a/gcd(a,b)*b;}

int main () {
  int n = in();
  vi dp(n+1);
  dp[0] = dp[1] = 1;
  srep(i,2,n+1) dp[i] = dp[i-1] + dp[i-2];
  cout << dp[n] << endl;
  return 0;
}

C++: #include <iostream>
#include <vector>

std::vector<int> dp(50, 1);

int fibonacci(int n) {
  if (n < 2)
    return dp.at(n);
  if (dp.at(n) == 1)
    dp.at(n) = fibonacci(n-1) + fibonacci(n-2);
  return dp.at(n);
}

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

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

#define N 45

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

C++: // -*- coding: utf-8 -*-
// Last-Updated : <2014/06/20 08:34:02 by samui>

#include <algorithm>
#include <iostream>
#include <cstdio>
#include <string>
#include <vector>
#include <climits>
#include <set>
#include <bitset>

#define MP(a, b) std::make_pair(a, b)
#define X first
#define Y second
#define rep(i,n) for(int (i) = 0; (i) < (int)(n); ++(i))
#define rer(i,l,n) for(int (i) = l; (i) <= (int)(n); ++(i))
#define reu(i,l,n) for(int (i) = l; (i) < (int)(n); ++(i))
template<typename T,typename U> inline void amin(T &x,U y){ if(y<x) x = y;}
template<typename T,typename U> inline void amax(T &x,U y){ if(x<y) x = y;}
template<typename TYPE,std::size_t SIZE>
std::size_t len(const TYPE (&array)[SIZE]){return SIZE;}

// 幾何ライブ
typedef long double LD;
const LD INF = 1e100;
const LD EPS = 1e-8;
const LD PI = acos(-1);
typedef std::pair<LD,LD> PLD;
typedef std::vector<std::pair<LD,LD>> VPLD2;

class Point{
public:
  LD x,y;
  Point(LD x = 0,LD y = 0): x(x),y(y){ };
  Point operator + (Point p) {return Point(x+p.x,y+p.y);};
  Point operator - (Point p) {return Point(x-p.x,y-p.y);};
  Point operator * (LD a) {return Point(x*a,y*a);};
  Point operator / (LD a) {return Point(x/a,y/a);};
  LD abs(){return std::sqrt(x*x+y*y);};
  Point unit(){LD u = abs();return Point(x,y)/u; };
};
typedef Point Vector2D;
LD dotProduct(const Vector2D a,const Vector2D b){
  return a.x*b.x+a.y*b.y;
}
LD crossProduct(const Vector2D a, const Vector2D b){return a.x*b.y-a.y*b.x;}
LD angle(Vector2D a,Vector2D b){
  return acos(dotProduct(a,b)/(a.abs()*b.abs()));
}
LD norm(Vector2D a){return a.x*a.x+a.y*a.y;}
Point projection(Point a1,Point a2,Point a3){
  // 点a3にたいする線分a1-a2上への射影をもとめる。
  LD d = dotProduct(a3-a1,a2-a1)/norm(a2-a1);
  return (a1+(a2-a1)*d);
}
Point reflection(Point a1,Point a2,Point a3){
  return (a3+(projection(a1,a2,a3)-a3)*2);
}
int ccw(Point a1,Point a2,Point a3){
  Vector2D a = a2-a1;
  Vector2D b = a3-a1;
  if(crossProduct(a,b) > EPS) return 1; // a1,a2,a3が反時計まわり
  if(crossProduct(a,b) < -EPS) return -1;//a1,a2,a3が時計まわり
  if(dotProduct(a,b) < -EPS) return +2; //a3--a1--a2 で直線
  if(norm(a)< norm(b)) return -2; // a1--a2--a3 の順で直線
  return 0; // a1--a3--a2の順番
}


int main(int argc, char *argv[])
{
  int a[45];
  a[0] = 0;
  a[1] = 1;
  reu(i,2,45){
    a[i] = a[i-1]+a[i-2];
  }
  int t;
  std::cin>>t;
  std::cout<<a[t]<<std::endl;

  return 0;
}