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  1. #include <bits/stdc++.h>
  2. using namespace std;
  3. #define ll long long
  4. #define int long long int
  5. #define ld long double
  6. #define all(x) x.begin(), x.end()
  7. #define sortall(x) sort(all(x))
  8. #define endl '\n'
  9. #define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
  10. template<class T>
  11. void printC (T Collection)
  12. {
  13.     for (auto&i:Collection)
  14.         cout << i << " \n";
  15.     cout << '\n';
  16. }
  17.  
  18. /*
  19.  * Think twice, code once
  20.  * Think of different approaches to tackle a problem: write them down.
  21.  * Think of different views of the problem. don't look from only one side.
  22.  * don't get stuck in one approach.
  23.  * common mistakes: - over_flow
  24.  *                  - out_of_bound index
  25.  *                  - infinite loop
  26.  *                  - corner cases
  27.  *                  - duplication counting.
  28. */
  29.  
  30. void solve()
  31. {
  32.     int n, k; cin >> n >> k;
  33.     if (k*(k+1)/2-(k-1) < n)
  34.     {
  35.         cout << -1;
  36.         return;
  37.     }
  38.     int l = 2, r = k, ans = -1;
  39.     while (l <= r)
  40.     {
  41.         int mid = (l+r)>>1;
  42.         if ([&](int)->bool
  43.         {
  44.             int have = k*(k+1)/2-1 - ((mid-1)*mid/2-1);
  45.             return have >= n;
  46.         }(mid))
  47.         {
  48.             ans = k-mid+1;
  49.             l = mid + 1;
  50.         }else
  51.             r = mid - 1;
  52.     }
  53.     cout << ans;
  54. }
  55.  
  56. int32_t main()
  57. {
  58.     // #ifndef ONLINE_JUDGE
  59.     //     freopen("input.txt", "r", stdin);
  60.     //     freopen("output.txt", "w", stdout);
  61.     //     freopen("Errors.txt", "w", stderr);
  62.     // #endif
  63.     fast
  64.     int t = 1;
  65.     // cin >> t;
  66.     while (t--)
  67.     {
  68.         solve();
  69.         if (t) cout << '\n';
  70.     }
  71.     cout << '\n';
  72.     return 0;
  73. }
Success #stdin #stdout 0.01s 5288KB
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https://ideone.com/3R1da1
language:
C++14 (gcc 8.3)
created:
14 days ago
visibility:
public

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