import core.bitop;
import std.algorithm;
import std.ascii;
import std.bigint;
import std.conv;
import std.functional;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.random;
import std.typecons;
import std.container;

alias sread = () => readln.chomp();
alias Point2 = Tuple!(long, "y", long, "x");

T lread(T = long)()
{
    return readln.chomp.to!T();
}

T[] aryread(T = long)()
{
    return readln.split.to!(T[])();
}

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}

void main()
{
    long n, m;
    scan(n, m);
    auto drink = new long[][](n, 2);
    foreach (ref e; drink)
    {
        e = aryread();
    }
    drink.sort();
    long count, maney, tmp;
    foreach (i, e; drink)
    {
        if (count + e[1] <= m)
        {
            count += e[1];
            maney += e[0] * e[1];
        }
        else
        {
            tmp = i;
            break;
        }
    }
    // writeln(maney);
    writeln(maney + (m - count) * drink[tmp][0]);
}