#include <iostream>
#include <fstream>

using namespace std;

int main() {
    int G, W, EGi, CGi, cTotal = 0, cost_total = 0;

    ifstream fin("energii.in");
    ofstream fout("energii.out");

    fin >> G >> W;

    for (int i = 0; i < G; i++) {
        fin >> EGi >> CGi;
        cTotal += EGi;
        cost_total += CGi;
    }

    if (cTotal < W) {
        fout << "-1" << endl;
    } else {
        int cProdusa = 0, cost_minim = 0;
        for (int i = 0; i < G; i++)
        {
            if (cProdusa >= W)
                {
                break;
                }
            EGi = min(W - cProdusa, EGi);
            cProdusa += EGi;
            cost_minim += EGi * CGi;
        }

        fout << cost_minim << endl;
    }

    fin.close();

    return 0;
}