Problem:
Score : 300 points

Problem Statement

Hearing that energy drinks increase rating in those sites, Takahashi decides to buy up M cans of energy drinks.

There are N stores that sell energy drinks. In the i-th store, he can buy at most B_i cans of energy drinks for A_i yen (the currency of Japan) each.

What is the minimum amount of money with which he can buy M cans of energy drinks?

It is guaranteed that, in the given inputs, a sufficient amount of money can always buy M cans of energy drinks.

Constraints

All values in input are integers.

1 \leq N, M \leq 10^5

1 \leq A_i \leq 10^9

1 \leq B_i \leq 10^5

B_1 + ... + B_N \geq M

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
\vdots
A_N B_N

Output

Print the minimum amount of money with which Takahashi can buy M cans of energy drinks.

Sample Input 1

2 5
4 9
2 4

Sample Output 1

12

With 12 yen, we can buy one drink at the first store and four drinks at the second store, for the total of five drinks. However, we cannot buy 5 drinks with 11 yen or less.

Sample Input 2

4 30
6 18
2 5
3 10
7 9

Sample Output 2

130

Sample Input 3

1 100000
1000000000 100000

Sample Output 3

100000000000000

The output may not fit into a 32-bit integer type.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp88 {
    class Program {
        static void Main(string[] args) {
            var st = Console.ReadLine().Split().Select(long.Parse).ToArray();
            long N = st[0];
            long M = st[1];

            var A = new long[N];
            var B = new long[N];

            var tupleList = new List<Tuple<long, long>>();

            for (int i = 0; i < N; i++) {
                var x = Console.ReadLine().Split().Select(long.Parse).ToArray();

                A[i] = x[0];
                B[i] = x[1];

                tupleList.Add(new Tuple<long, long>(A[i], B[i]));
            }

            tupleList.Sort((a, b) => a.Item1.CompareTo(b.Item1));

            //これまでに買った本数
            long num = 0;
            //あと何本？
            long remain = M;

            //今何番目に安いやつを買っているか
            int buyIndex = 0;

            //これまでに買った金額
            long ans = 0;

            while (num < M) {
                remain = M - num;
                if (tupleList[buyIndex].Item2 >= remain) {
                    ans += tupleList[buyIndex].Item1 * remain;
                    num += remain;
                } else {
                    ans += tupleList[buyIndex].Item1 * tupleList[buyIndex].Item2;
                    num += tupleList[buyIndex].Item2;
                }
                buyIndex++;
            }

            Console.WriteLine(ans);
        }
    }
}