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# ��ˮö�١�#44 A. Triangular numbers

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A. Triangular numbers

time limit per test
2 seconds

memory limit per test
256 megabytes

input
standard input

output
standard output

A triangular number is the number of dots in an equilateral triangle uniformly filled with dots. For example, three dots can be arranged in a triangle; thus three is a triangular number. The?n-th triangular number is the number of dots in a triangle with?n?dots on a side.?. You can learn more about these numbers from Wikipedia (http://en.wikipedia.org/wiki/Triangular_number).

Your task is to find out if a given integer is a triangular number.

Input

The first line contains the single number?n?(1?��?n?��?500) ? the given integer.

Output

If the given integer is a triangular number output?YES, otherwise output?NO.

Sample test(s)

input

1

output

YES

input

2

output

NO

input

3

output

YES

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code:

`#include #include #include #include #include #include #include #include #include #include #include using namespace std;bool judge(int n){ for(int i=1;;i++) { if(i*(i+1)/2==n)return true; if(i*(i+1)/2 >n)return false; } return false;}int main(){ int n; cin>>n; cout<< (judge(n)?"YES":"NO")< return 0;}`

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