using Yorukatsu042.Questions;
using Yorukatsu042.Extensions;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace Yorukatsu042.Questions
{
    /// <summary>
    /// ABC121 C
    /// </summary>
    public class QuestionB : AtCoderQuestionBase
    {
        public override IEnumerable<object> Solve(TextReader inputStream)
        {
            var nm = inputStream.ReadIntArray();
            var storeCounts = nm[0];
            var needed = nm[1];

            var stores = new Store[storeCounts];
            for (int i = 0; i < stores.Length; i++)
            {
                var ab = inputStream.ReadIntArray();
                stores[i] = new Store(ab[0], ab[1]);
            }

            Array.Sort(stores);

            long total = 0;
            foreach (var store in stores)
            {
                var buy = Math.Min(store.Stock, needed);
                total += (long)store.Price * buy;
                needed -= buy;
                if (needed == 0)
                {
                    break;
                }
            }

            yield return total;
        }

        struct Store : IComparable<Store>
        {
            public int Price { get; }
            public int Stock { get; }

            public Store(int price, int stock)
            {
                Price = price;
                Stock = stock;
            }

            public int CompareTo(Store other) => Price.CompareTo(other.Price);
        }
    }
}


namespace Yorukatsu042
{
    class Program
    {
        static void Main(string[] args)
        {
            IAtCoderQuestion question = new QuestionB();    // 問題に合わせて書き換え
            var answers = question.Solve(Console.In);
            foreach (var answer in answers)
            {
                Console.WriteLine(answer);
            }
        }
    }
}

#region Base Classes

namespace Yorukatsu042.Questions
{

    public interface IAtCoderQuestion
    {
        IEnumerable<object> Solve(string input);
        IEnumerable<object> Solve(TextReader inputStream);
    }

    public abstract class AtCoderQuestionBase : IAtCoderQuestion
    {
        public IEnumerable<object> Solve(string input)
        {
            var stream = new MemoryStream(Encoding.Unicode.GetBytes(input));
            var reader = new StreamReader(stream, Encoding.Unicode);

            return Solve(reader);
        }

        public abstract IEnumerable<object> Solve(TextReader inputStream);
    }

}

#endregion

#region Extensions

namespace Yorukatsu042.Extensions
{
    internal static class TextReaderExtensions
    {
        internal static int ReadInt(this TextReader reader) => int.Parse(ReadString(reader));
        internal static long ReadLong(this TextReader reader) => long.Parse(ReadString(reader));
        internal static double ReadDouble(this TextReader reader) => double.Parse(ReadString(reader));
        internal static string ReadString(this TextReader reader) => reader.ReadLine();

        internal static int[] ReadIntArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(int.Parse).ToArray();
        internal static long[] ReadLongArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(long.Parse).ToArray();
        internal static double[] ReadDoubleArray(this TextReader reader, char separator = ' ') => ReadStringArray(reader, separator).Select(double.Parse).ToArray();
        internal static string[] ReadStringArray(this TextReader reader, char separator = ' ') => reader.ReadLine().Split(separator);
    }
}

#endregion

Measured memory_bytes: 18488