#include <iostream>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <string>
#include <cstring>
using namespace std;

const int MAX_N = 10005;
struct Node{int first, second, pos, cost;} p[MAX_N], p2[MAX_N];
int n, s;

bool cmp(Node a, Node b){return a.first < b.first;}

void solve()
{
    sort(p2, p2+n, cmp);
    for (int i=n-1; i>=0; --i)
    {
        int j=0;
        int max_cost = 0;
        while (j < n && p[i].first - p2[j].first > s)
        {
            int dia1 = p[i].first - p2[j].first;//生产成本之差
            int dia2 = (p[i].pos - p2[j].pos) * s;//储存费用
            if (dia2 > 0 && (dia1 - dia2) > max_cost)
            {
                max_cost = dia1 - dia2;
            }
            ++j;
        }
        p[i].cost = (p[i].first - max_cost) * p[i].second;
    }
    long long res = 0;
    for (int i=0; i<n; ++i) res += p[i].cost;
    printf("%lld\n", res);
    return;
}

int main()
{
    while (scanf("%d %d", &n, &s) != EOF)
    {
        for (int i=0; i<n; ++i)
        {
            scanf("%d %d", &p[i].first, &p[i].second);
            p2[i].first = p[i].first;
            p2[i].second = p[i].second;
            p2[i].pos = p[i].pos = i;
        }
        solve();
    }
    return 0;
}