Paired Up (vec)

#include <iostream>
#include <algorithm>
#include <vector>
#define ll long long
using namespace std;

int main()
{
    ll n, V=0;
    cin>>n;
    vector< pair<ll, ll> > v;

    ll x, y;
    for(ll i=0; i<n; i++)
    {
        cin>>x>>y;
        v.push_back( {y,x} );
        V+=x;
    }

    sort(v.begin(), v.end());

    /**for(ll i=0; i<v.size(); i++)
        cout<<v[i].first<<"-"<<v[i].second<<" ";
    cout<<endl;**/

    ll ini=0;
    ll fin=v.size()-1;

    ll suma, may=(-1);

    for(ll i=0; i<V/2; i++)
    {
        /*if (ini<0 || ini >=v.size())
            cout<<"ERROR";
        if (fin<0 || fin >=v.size())
            cout<<"ERROR";*/

        suma = v[ini].first + v[fin].first;
        if (suma > may)
            may = suma;

        v[ini].second--;
        v[fin].second--;


        if (v[ini].second == 0)
            ini++;

        if (v[fin].second == 0)
            fin--;
    }
    cout<<may;

    return 0;
}