Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func read() string {
	sc.Scan()
	return sc.Text()
}

func main() {
	sc.Split(bufio.ScanLines)
	n, _ := strconv.Atoi(read())
	sum := 1
	for i := n; i > 0; i-- {
		sum *= i

	}
	fmt.Println(sum)
}

C++: // need
#include <iostream>
#include <algorithm>

// data structure
#include <bitset>
//#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
//#include <array>
//#include <tuple>
//#include <unordered_map>
//#include <unordered_set>
#include <complex>
//#include <deque>
#include<valarray>

// stream
//#include <istream>
//#include <sstream>
//#include <ostream>
#include <fstream>

// etc
#include <cassert>
#include <functional>
#include <iomanip>
//#include <typeinfo>
//#include <chrono>
#include <random>
#include <numeric>

#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int i=0; i<n; ++i){MACRO_VEC_ROW_Scan(i, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest&...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest&...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define OUT(d) std::cout<<d;
#define FOUT(n, d) std::cout<<std::fixed<<std::setprecision(n)<<d;
#define SOUT(n, c, d) std::cout<<std::setw(n)<<std::setfill(c)<<d;
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(i, n) std::cout<<(i + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& r:c)for(auto& i:r)std::cin>>i;
#define ALL(a) (a).begin(),(a).end()
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define RREP(i, n) for(int i=(n)-1;i>=0;--i)
#define FORLL(i, a, b) for(ll i=ll(a);i<ll(b);++i)
#define RFORLL(i, a, b) for(ll i=ll(b)-1;i>=ll(a);--i)
#define REPLL(i, n) for(ll i=0;i<ll(n);++i)
#define RREPLL(i, n) for(ll i=ll(n)-1;i>=0;--i)
#define PAIR std::pair<int, int>
#define PAIRLL std::pair<ll, ll>
#define IN(a, x, b) (a<=x && x<b)
#define SHOW(d) {std::cerr << #d << "\t:" << d << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWPAIR(p) {std::cerr << #p << "\t:(" << p.first << ",\t" << p.second << ")\n";}
#define SHOWPAIRVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr<<'('<<yyy.first<<", "<<yyy.second<<") ";}std::cerr << "\n";}}
#define SHOWPAIRVECTOR(v) {for(const auto& xxx:v){std::cerr<<'('<<xxx.first<<", "<<xxx.second<<") ";}std::cerr<<"\n";}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t:";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}
template<typename T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<typename T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
#define EXCEPTION(msg) throw std::string("Exception : " msg " [ in ") + __func__ + " : " + std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg) try {if (cond) EXCEPTION(msg);}catch (std::string s) {std::cerr << s << std::endl;}
//void CHECKTIME(std::function<void()> f) { auto start = std::chrono::system_clock::now(); f(); auto end = std::chrono::system_clock::now(); auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start)).count(); std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n"; }

#define int ll
using ll = long long;
using ull = unsigned long long;
constexpr int INFINT = 1 << 30;                          // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-7;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;

int fact(int n) {
	return !n ? 1 : n * fact(n - 1);
}

signed main() {
	INIT;
	VAR(int, n);
	OUT(fact(n))BR;
	return 0;
}

C++: #include<stdio.h>
#include <string>
#include <iostream>
#include <vector>
#include <sstream>
#include<math.h>
#include <algorithm>
using namespace std;


int factorial_recursive(int n);

int main(void)
{
  int n;
   int result1;


cin>>n;
if(0<n&&n<21) ;
else return 0;
	result1 = factorial_recursive(n);

	cout<<result1<<endl;

}
int factorial_recursive(int n) {
		if (n > 0) {
        return factorial_recursive(n - 1) * n;
    } else {
        return 1;
    }
}

C++: #include<iostream>
using namespace std;
int main(){
	unsigned __int64 a=1;
	int i,n;
	cin>>n;
	for(i=n;i>0;i--){
		a*=i;
	}
	cout<<a<<endl;
	return 0;
}

C++: #include <iostream>

using namespace std;

int main() {
  long long n, r = 1;
  cin >> n;
  for (; n > 1; n--)
    r *= n;
  cout << r << endl;
  return 0;
}

C++: #include<iostream>

using namespace std;

int main()
{
  int n;
  while(cin >> n){
    int i = n;
    while(i-- != 1) n *= i;
    cout << n << endl;;
  }
}

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

int main(void)
{
	int n;
	int i;
	int anser = 1;
	
	cin >> n;
	
	for (i = 1; i <= n; i++){
		anser *= i;
	}
	
	cout << anser;
	
	return (0);
}

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

int main() {
	long n,a;
	cin>>n;
	a = n;
	while(true){
		n = n-1;
		if (n == 0)break;
		a = a*n;
	}cout<<a<<endl;
}

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

int main()
{
	int n;
	scanf("%d", &n);
	int Fraction = 1;
	for(int i = 0; i < n; i++){
		Fraction *= n-i;
	}
	printf("%d\n", Fraction);
	return 0;
}

C++: #include<iostream>
#include<iomanip>
#include<algorithm>
#include<cctype>
#include<cmath>
#include<cstdio>
#include<set>
#include<stack>
#include<string>
#include<sstream>
#include<map>
#include<queue>
#include<vector>

using namespace std;

int main() {
	int n;
	cin >> n;
	1 <= n <= 20;
	long long ans = n;
	for(int i=n-1;i>1;i--){
		ans *= i;
	}
	cout << ans << endl;
	return 0;
}

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

int fact(int n){
	if(n==1)return 1;
	else return n*fact(n-1);
}

int main(int argc, char *argv[]){
	int n;
	cin>>n;
	cout<<fact(n)<<endl;
    return 0;
}

C++: #include<iostream>
unsigned long long factorial(int n)
{
	if (n == 1)
	{
		return 1;
	}
	else
	{
		return n * factorial(n - 1);
	}
}
int main()
{
	int n;
	std::cin >> n;
	std::cout << factorial(n) << std::endl;
	return 0;
}

C++: #include<iostream>
int input=0;

long long factorial(int n)
{
	if(n==0)
		return 1;
  return (n == 1 || n == 0) ? 1 : factorial(n - 1) * n;
}

int main()
{
	do
	{	
		std::cin>>input;
		if(input<0||input>20)
			break;
		std::cout<<factorial(input);
	}while(true);


	return 0;
}

C++: #include<iostream>
main(){int n,i;std::cin>>n;for(i=n;--n;)i*=n;std::cout<<i;}