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  1. #include <bits/stdc++.h>
  2. #define nmax 10000
  3. using namespace std;
  4. long long n,coun;
  5. void ip()
  6. {
  7.     cin >> n;
  8. }
  9. void pc()
  10. {
  11.     if (n!=1)
  12.     {
  13.         coun=1;
  14.         while(n==n)
  15.         {
  16.             coun++;
  17.             if ((coun*(coun-1))/2==n)
  18.             {
  19.                 cout << "YES";
  20.                 break;
  21.             }
  22.             if ((coun*(coun-1))/2>n)
  23.             {
  24.                 cout << "NO";
  25.                 break;
  26.             }
  27.         }
  28.     }
  29.     else cout << "YES";
  30. }
  31. int main()
  32. {
  33.     ios_base::sync_with_stdio();
  34.     ip();
  35.     pc();
  36.     return 0;
  37. }
Success #stdin #stdout 0.11s 2744KB
comments (?)
stdin
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555555561111111
stdout
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YES
https://ideone.com/0TWsYi
language:
C++ 4.3.2 (gcc-4.3.2)
created:
6 years ago
visibility:
public

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