Funky Numbers - CF 192A Two Pointers

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll n;

ll fun(ll l,ll r)
{
    ll val=l*l+r*r+l+r;
    if(val==n)
        return 1;
    if(val<n)
        return -1;
    return 0;
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    ll t;
    t=1;
    while(t--)
    {
        int flag=0;
        ll left=1;
        cin >> n;
        n*=2;
        ll right=(ll)sqrt(2*n)+1;
        while(left<=right)
        {
            if(fun(left,right)==1)
            {
                flag=1;
                break;
            }
            else if(fun(left,right)==-1)
            {
                left++;
            }
            else
            {
                right--;
            }
        }
    if(flag)
        cout<<"YES";
    else
        cout<<"NO";

    }

    return 0;
}