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  1. #include <bits/stdc++.h>
  2. #define VuDucNam ios_base::sync_with_stdio(false);
  3. #define Lop9a6 cin.tie(NULL);
  4. #define THCSPhamVanDong cout.tie(NULL);
  5. #define ll long long
  6. #define el cout << '\n'
  7. #define sz(a) (ll) a.size()
  8. #define all(a) a.begin(), a.end()
  9. #define fi first
  10. #define se second
  11. #define fill(a, x) memset(a, x, sizeof(a))
  12. #define file(name)                         \
  13.     if (fopen(name ".inp", "r"))           \
  14.     {                                      \
  15.         freopen(name ".inp", "r", stdin);  \
  16.         freopen(name ".out", "w", stdout); \
  17.     }
  18. #define TIME (1.0 * clock() / CLOCKS_PER_SEC)
  19. #define RUNTIME cerr << "\nRuntime: " << TIME << "s.\n"
  20.  
  21. using namespace std;
  22.  
  23. const int N5 = 1e5 + 5;
  24. const ll MOD = 1e9 + 7;
  25. const ll inf = LLONG_MAX;
  26.  
  27. vector<bool> prime(10000001,true);
  28. vector<int> nto;
  29. int n,k;
  30. vector<ll> ans;
  31.  
  32. void sang()
  33. {
  34.     prime[0]=prime[1]=false;
  35.     for(int i=2;i<=sqrt(1e7);++i){
  36.         if(prime[i]){
  37.             for(int j=i*i;j<=1e7;j+=i) 
  38.                 prime[j]=false;
  39.         }
  40.     }
  41.     for(int i=2;i<=sqrt(1e9);++i){
  42.         if(prime[i]) nto.push_back(i);
  43.     }
  44. }
  45.  
  46. bool check(ll n)
  47. {
  48.     for(int i=0;1LL*nto[i]*nto[i]<=n && i<(int)nto.size();++i){
  49.         if(n%nto[i]==0)
  50.             return false;
  51.     }
  52.     return true;
  53. }
  54.  
  55. void solve()
  56. {
  57.     sang();
  58.     cin >> n >> k;
  59.     for(int i=1;i<=n;++i){
  60.         ll x;
  61.         cin >> x;
  62.         if(x<=1e7){
  63.             if(prime[x]) ans.push_back(x);
  64.         }else{
  65.             if(check(x)) ans.push_back(x);
  66.         }
  67.     }
  68.     sort(all(ans));
  69.     if((int)ans.size()<k)
  70.         cout << -1;
  71.     else 
  72.         cout << ans[k-1];
  73.     el;
  74. }
  75.  
  76. int main()
  77. {
  78.     VuDucNam Lop9a6 THCSPhamVanDong
  79.     // file();
  80.     solve();
  81.     RUNTIME;
  82.     return 0;
  83. }
  84.  
Success #stdin #stdout #stderr 0.06s 5288KB
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stdin
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10 3
2 8 5 3 7 10 11 4 6 13
stdout
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5
stderr
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Runtime: 0.056093s.
https://ideone.com/XDSZ5M
language:
C++ (gcc 8.3)
created:
5 days ago
visibility:
public

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