#include <iostream>
#include <fstream>

using namespace std;

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

int n, gm;
int castig = 0;

struct obiect
{
  int g,v;
  int c;
} o[20000];

void citire()
{
    int i;
    fin >> n >> gm;
    for(i = 1; i <= n; i++)
    {
        fin >> o[i].g >> o[i].v;
        o[i].c = o[i].v;
    }
}

void sortare()
{
    int i, j;
    for(i = 1; i < n; i++)
        for(j = i + 1; j <= n; j++)
            if(o[i].c < o[j].c)
                swap(o[i], o[j]);
}

int alegere()
{
    int i;
    for(i = 1; i <= n && gm != 0; i++)
    {
        if(o[i].g <= gm)
        {
            gm -= o[i].g;
            castig += o[i].v;
        }
        else
        {
            castig += o[i].c * gm;
            gm = 0;
        }
    }
    return castig;
}

int main()
{
    citire();
    sortare();
    fout << alegere() << "\n";
    fin.close();
    fout.close();
    return 0;
}