#include #define syosu(x) fixed< P; typedef pair pdd; typedef pair pll; typedef vector vi; typedef vector vvi; typedef vector vd; typedef vector vvd; typedef vector vl; typedef vector vvl; typedef vector vs; typedef vector

vp; typedef vector vvp; typedef vector vpll; typedef pair pip; typedef vector vip; const int inf=1<<30; const ll INF=1ll<<60; const double pi=acos(-1); const double eps=1e-8; const ll mod=1e9+7; const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1}; ll n; int main(){ cin>>n; ll l=1,r=2000000001; while(r-l>1){ ll m=(l+r)/2; if(m*(m+1)/2<=n) l=m; else r=m; } if(l*(l+1)/2==n) cout<<"YES"<