#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n,t,l,e;
    cin >> n >> t;
    multimap<int, int> monster;
    for (int i = 0; i < n; ++i)
    {
        cin >> l >> e;
        monster.insert(pair<int, int>(l,e));
    }
    int now = 0, res = 0;
    priority_queue<int, vector<int>, greater<int> > pq;
    for (auto i = monster.begin(); i != monster.end();)
    {
        if ((i -> first)/t == now)
        {
            if (pq.size() < now + 1)
                pq.push(i -> second);
            else if (i -> second > pq.top())
            {
                pq.pop();
                pq.push(i -> second);
            }
            i++;
        }
        else
        {
            now = (i -> first)/t;
        }
    }
    while (!pq.empty())
    {
        res += pq.top();
        pq.pop();
    }
    cout << res << endl;
}