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18 lines
280 B
C++
18 lines
280 B
C++
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#include <bits/stdc++.h>
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using namespace std;
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int main() {
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int n;
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cin >> n;
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for (int i = 1; i <= n; i++) {
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if ((i + 1) * i * 0.5 == n) {
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cout << "YES" << endl;
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goto end;
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}
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}
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cout << "NO" << endl;
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end:;
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return 0;
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}
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