#include //#include using namespace std; #define INF 1.1e9 #define LINF 1.1e18 #define FOR(i,a,b) for (int i=(a);i<(b);++i) #define REP(i,n) FOR(i,0,n) #define ALL(v) (v).begin(),(v).end() #define pb push_back #define pf push_front #define fi first #define se second #define BIT(x,n) bitset(x) #define PI 3.14159265358979323846 typedef long long ll; typedef pair P; typedef pair PP; //----------------------------------------------------------------------------- ll n; int main() { cin.tie(0); ios::sync_with_stdio(false); cin>>n; for(ll i=1;i*(i+1)<=2*n;i++) { if((i+1)*i==2*n) { cout<<"YES"<