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++: #ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdalign>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif
#define y0 qvya13579
#define y1 qvyb24680
#define j0 qvja13579
#define j1 qvjb24680
#define next qvne13579xt
#define prev qvpr13579ev
#define INF 1000000007
#define PI acos(-1.0)
#define endl "\n"
#define IOS cin.tie(0);ios::sync_with_stdio(false)
#define M_P make_pair
#define P_B push_back
#define U_B upper_bound
#define L_B lower_bound
#define P_Q priority_queue
#define FIR first
#define SEC second
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define REP_R(i,n) for(int i=((int)(n)-1);i>=0;--i)
#define FOR(i,m,n) for(int i=((int)(m));i<(int)(n);++i)
#define FOR_R(i,m,n) for(int i=((int)(m)-1);i>=(int)(n);--i)
#define ALL(v) (v).begin(),(v).end()
#define RALL(v) (v).rbegin(),(v).rend()
#define SIZ(x) ((int)(x).size())
#define COUT(x) cout<<(x)<<endl
#define CIN(x) cin>>(x)
#define CIN2(x,y) cin>>(x)>>(y)
#define CIN3(x,y,z) cin>>(x)>>(y)>>(z)
#define CIN4(x,y,z,w) cin>>(x)>>(y)>>(z)>>(w)
#define SCAND(x) scanf("%d",&(x))
#define SCAND2(x,y) scanf("%d%d",&(x),&(y))
#define SCAND3(x,y,z) scanf("%d%d%d",&(x),&(y),&(z))
#define SCAND4(x,y,z,w) scanf("%d%d%d%d",&(x),&(y),&(z),&(w))
#define SCANLLD(x) scanf("%lld",&(x))
#define SCANLLD2(x,y) scanf("%lld%lld",&(x),&(y))
#define SCANLLD3(x,y,z) scanf("%lld%lld%lld",&(x),&(y),&(z))
#define SCANLLD4(x,y,z,w) scanf("%lld%lld%lld%lld",&(x),&(y),&(z),&(w))
#define PRINTD(x) printf("%d\n",(x))
#define PRINTLLD(x) printf("%lld\n",(x))
typedef long long int lli;
using namespace std;

int ctoi(char c)
{
  if( c >= '0' and c <= '9' )
    {
      return (int)(c-'0');
    }

  return -1;
}

int alphabet_pos(char c) 
{
  if( c >= 'a' and c <= 'z' )
    {
      return (int)(c-'a');
    }

  return -1;
}


int alphabet_pos_capital(char c)
{
  if( c >= 'A' and c <= 'Z' )
    {
      return (int)(c-'A');
    }

  return -1;
}


vector<string> split(string str, char ch)
{
  int first = 0;
  int last = str.find_first_of(ch);
  
  if(last == string::npos)
    {
      last = SIZ(str);
    }

  vector<string> result;

  while( first < SIZ(str) )
    {
      string Ssubstr(str, first, last - first);
      result.push_back(Ssubstr);
      first = last + 1;
      last = str.find_first_of(ch, first);

      if(last == string::npos)
	{
	  last = SIZ(str);
	}
    }
  
  return result;
}

/*--------------- the end of the template --------------------*/


int fibonacci( int n , vector<int>& list)
{
  if( n == 0 or n == 1 )
    {
      return list[n] = 1;
      
    }

  if( list[n] > 0 )
    {
      return list[n];
      
    }

  return list[n] = fibonacci(n-1,list) + fibonacci(n-2,list);
  
}





int main()
{
  //IOS; /* making cin faster */
  int n;
  SCAND(n);
  vector<int> list(n,-1);

  
  PRINTD(fibonacci(n,list));
  
  
  

}

C++: #include <string>
#include <iostream>
#include <math.h>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <set>
#include <map>
typedef long long ll;
const int mod=1000000007;

#define lol(i,n) for(int i=0;i<n;i++)

using namespace std;
int f[50];

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

C++: #include<iostream>
using namespace std;
#define MAX 44

int main(){
  int n;
  int F[50];
  F[0] = F[1] = 1;
  
  cin >> n;

  for(int i=2;i<n+1;i++){
    F[i]=F[i-1]+F[i-2];
  }

  cout << F[n] << endl;

  return 0;
}

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]=fib(n-1)+fib(n-2);
}

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

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

int main() {

  int n;
  cin>>n;

  int a[n+1];
  a[0] = 1;
  a[1] = 1;

  for(int i = 2; i <= n; i++) {
    a[i] = a[i-1] + a[i-2];
  }

  cout<<a[n]<<endl;
  
  return 0;
}

C++: //
// Created by 李青坪 on 2018/10/28.
//

#include <cstdio>
#include <algorithm>
#define MAXN 44
#define NIL -1
using namespace std;

int value[MAXN];

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

int main(){
    fill(value, value+MAXN, NIL);
    int n;
    scanf("%d", &n);
    printf("%d\n", fibonacci(n));
}

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

#if 0
int Fibonatch(int result[], int num)
{
    int ret = result[num];

    if (ret == 0) {
        if (num > 1) {
            ret = Fibonatch(result, num - 1) + Fibonatch(result, num - 2);
        }
        else {
            ret = 1;
        }
        result[num] = ret;
    }

    return ret;
}

int main(void)
{
    int num;
    int ret[45] = {0};
    int result;

    scanf("%d", &num);
    result = Fibonatch(ret, num);

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

    return 0;
}
#else

int Fibonatch(int num)
{
    int i;
    int result[45];

    result[0] = result[1] = 0;
    for (i = 2; i <= num; i++) {
        result[i] = result[i - 1] + result[i - 2];
    }

    return result[num];
}

int main(void)
{
    int num;
    int result;

    scanf("%d", &num);

    result = Fibonatch(num);
    printf("%d\n", result);

    return 0;
}

#endif

C++: #include <bits/stdc++.h>
using namespace std;
void fib(int *a, int n)
{
    a[0] = 1;
    a[1] = 1;
    for (int i = 2; i <= n; ++i)
    {
        a[i] = a[i - 2] + a[i - 1];
    }
}
int main()
{
    int n;
    cin >> n;
    int a[n + 1] = {};
    fib(a, n);
    cout << a[n] << endl;
}