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  1. #include <bits/stdc++.h>
  2.  
  3. using namespace std;
  4. long long n,k;
  5. int main()
  6. {
  7. 	cin >> k;
  8. 	n=sqrt(2*k);
  9. 	if((n*(n+1)/2)==k) cout << "YES";
  10. 	else cout << "NO";
  11. 	return 0;
  12. }
  13.  
Success #stdin #stdout 0s 2744KB
comments (?)
stdin
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555555561111111
stdout
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YES
https://ideone.com/rZAlwJ
language:
C++ 4.3.2 (gcc-4.3.2)
created:
6 years ago
visibility:
public

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