#include <fstream>
#include <vector>

using namespace std;

#define _int64 unsigned long long

int main(int argc, char** argv) {
    ifstream fin("factory.in");
    ofstream fout("factory.out");

    _int64 n, s;
    fin >> n >> s;
    _int64 c, y;
    vector<pair<_int64, _int64> > data;

    for (int i = 0; i < n; ++i) {
        fin >> c >> y;
        data.push_back(make_pair(c, y));
    }

    _int64 cost = 0;
    for (int i = 0; i < n; ++i) {
        int j = 0;
        for (; j + i < n; ++j) {
            if (data[i].first + j * s <= data[j + i].first) {
                cost += (data[i].first + j * s) * data[j + i].second;
                continue;
            }
            break;
        }
        i += (j - 1);
    }

    fout << cost << endl;

    return 0;
}