using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;

namespace AtCoder.ABC121 {
    class C {
        static void Main() => CustomMain();

        class Shop {
            public int money;
            public int stock;
        }

        static void Solve() {
            int n, m;
            ReadMulti(out n, out m);
            var shops = new Shop[n];
            for (int i = 0; i < n; i++) {
                var t = ReadIntArray();
                shops[i] = new Shop {money = t[0], stock = t[1]};
            }

            var ss = shops.OrderBy(x => x.money).ToList();

            long ans = 0;
            var mm = 0;
            for (int i = 0; i < n; i++) {
                var buyNum = Math.Min(m - mm, ss[i].stock);
                ans += (long) ss[i].money * buyNum;
                mm += buyNum;
            }

            WriteAnswer(ans);
        }


        static long GCD(long a, long b) {
            return b != 0 ? GCD(b, a % b) : a;
        }

        static void CustomMain() {
#if DEBUG
            // ファイルからテストケースを読み込む
            global::Common.ChangeStdIn();
            // 時間計測
            var stopwatch = new Stopwatch();
            stopwatch.Start();
#endif
            // 出力高速化
            var streamWriter = new StreamWriter(Console.OpenStandardOutput()) {AutoFlush = false};
            Console.SetOut(streamWriter);
            // 回答
            Solve();
#if DEBUG
            stopwatch.Stop();
            Console.WriteLine(stopwatch.Elapsed);
#endif
            Console.Out.Flush();
        }

        static string ReadString() => Console.ReadLine().Trim();
        static int ReadInt() => int.Parse(ReadString());
        static long ReadLong() => long.Parse(ReadString());
        static float ReadFloat() => float.Parse(ReadString());
        static double ReadDouble => double.Parse(ReadString());

        static string[] ReadStringArray() => ReadString().Split(' ');
        static int[] ReadIntArray() => Array.ConvertAll(ReadString().Split(' '), int.Parse);
        static long[] ReadLongArray() => Array.ConvertAll(ReadString().Split(' '), long.Parse);
        static float[] ReadFloatArray() => Array.ConvertAll(ReadString().Split(' '), float.Parse);
        static double[] ReadDoubleArray() => Array.ConvertAll(ReadString().Split(' '), double.Parse);

        static T TypeConversion<T>(string s) {
            if (typeof(T) == typeof(int)) {
                return (T) Convert.ChangeType(int.Parse(s), typeof(T));
            } else if (typeof(T) == typeof(long)) {
                return (T) Convert.ChangeType(long.Parse(s), typeof(T));
            } else if (typeof(T) == typeof(double)) {
                return (T) Convert.ChangeType(double.Parse(s), typeof(T));
            } else {
                return (T) Convert.ChangeType(s, typeof(T));
            }
        }

        static void ReadMulti<T, U>(out T a, out U b) {
            var str = ReadStringArray();
            a = TypeConversion<T>(str[0]);
            b = TypeConversion<U>(str[1]);
        }

        static void ReadMulti<T, U, V>(out T a, out U b, out V c) {
            var str = ReadStringArray();
            a = TypeConversion<T>(str[0]);
            b = TypeConversion<U>(str[1]);
            c = TypeConversion<V>(str[2]);
        }

        static void WriteAnswer(object ans) => Console.WriteLine(ans);

        static bool IsOdd(long num) {
            return (num & 1) == 1;
        }

        static bool IsEven(long num) {
            return (num & 1) == 0;
        }
    }
}

Measured memory_bytes: 22228