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++14 (Clang 3.8.0) (judge verdict: Accepted)

#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <functional>
#include <bitset>
#include <cmath>
#include <stack>
typedef long long ll;
using namespace std;

ll GCD(ll a, ll b) { return b ? GCD(b, a%b) : a; }



int main()
{
    int N,M;
    cin >> N >> M;
    vector<pair<int, int> > AB(N);
    for(int i = 0; i < N; i++)
    {
        int A,B;
        cin >> A >> B;
        AB[i].first = A;
        AB[i].second = B;
    }
    sort(AB.begin(),AB.end());
    ll money = 0;
    ll juice = 0;
    int i = 0;
    for(int i = 0; i < N && M > 0; i++)
    {
        money += AB[i].first * min(M,AB[i].second);
        M -= min(M,AB[i].second);

    }
    cout << money << endl;
}