#include <bits/stdc++.h> using namespace std; typedef long long ll; template <class A, class B> bool maximize(A &a, const B b) { if (a < b) { a = b; return true; } return false; } template <class A, class B> bool minimize(A &a, const B b) { if(a > b) { a = b; return true; } return false; } using pII = pair <int, int>; using vI = vector <int>; using vL = vector <long long>; using pLI = pair <long long, int>; #define BIT(mask, i) ((mask >> (i)) & 1) #define MASK(a) (1LL<<(a)) #define FOR(i, a, b) for(int i = a; i <= (int)b; i++) #define FORD(i, a, b) for(int i = a; i >= (int)b; i--) #define fi first #define se second #define pb push_back #define all(a) a.begin(), a.end() #define sz(a) (int)a.size() const int mod = 1e9 + 7; const int maxn = 2e5 + 5; void add (int &x, const int y) { x+= y; if (x >= mod) x-= mod; } int n, k; int fact[maxn], inv_fact[maxn]; int iPow (int x, int y) { if (y == 0) return 1; if (y == 1) return x; int c = iPow (x, y / 2); if (y & 1) return 1LL * c * c % mod * x % mod; else return 1LL * c * c % mod; } int C (int n, int k) { if (k > n) return 0; return 1LL * fact[n] * inv_fact[k] % mod * inv_fact[n - k] % mod; } namespace sub1 { int a[15]; void solve () { FOR(i, 1, n) a[i] = i; int ans = 0; do { int cnt = 0; vector <int> dd(n + 2, 0); FOR(i, 1, n) { if (dd[a[i]] == 0) { cnt++; int last = - 1; FOR(j, i, n) if (!dd[a[j]] && a[j] >= last) { dd[a[j]] = 1; last = a[j]; } else break; } } if (cnt == k) ans++; } while (next_permutation(a + 1, a + 1 + n)); cout << ans; } } namespace sub23 { int dp[3005][3005]; void solve () { dp[1][1] = 1; FOR(i, 1, n) FOR(j, 1, k) { /// create new group if (j < k) { add(dp[i + 1][j + 1], dp[i][j]); add(dp[i + 1][j + 1], 1LL * dp[i][j] * (i - j) % mod); } /// join group add (dp[i + 1][j], 1LL * dp[i][j] * j % mod); } cout << dp[n][k]; } } void process () { cin >> n >> k; if (n <= 10) { return sub1::solve(); } if (n <= 3000) { return sub23::solve(); } } int main () { ios_base::sync_with_stdio(0); cin.tie(0); #define taskname "kieuoanh" if (fopen(taskname".inp", "r")) { freopen(taskname".inp", "r", stdin); freopen(taskname".out", "w", stdout); } fact[0] = 1; FOR(i, 1, maxn - 5) fact[i] = 1LL * fact[i - 1] * i % mod; inv_fact[maxn - 5] = iPow(fact[maxn - 5], mod - 2); FORD(i, maxn - 6, 0) inv_fact[i] = 1LL * inv_fact[i + 1] * (i + 1) % mod; int tc = 1; /// cin >> tc; while (tc--) process(); return 0; }

asceticism.cpp: In function 'int main()':
asceticism.cpp:122:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  122 |         freopen(taskname".inp", "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
asceticism.cpp:123:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  123 |         freopen(taskname".out", "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~