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 <vector>

using namespace std;

using ll = long long int;

struct entry {
  ll price;
  ll count;

  bool operator<(const entry &a) const {
    return price < a.price;
  }
};

// ll a[100000 + 1];
// ll b[100000 + 1];

vector<entry> hoge;

int main() {
  int n, m;
  cin >> n >> m;

  for (int i = 0; i < n; i++) {
    entry e;
    cin >> e.price >> e.count;
    hoge.push_back(e);
  }

  sort(hoge.begin(), hoge.end());

  ll money = 0;
  int index = 0;
  while (m > 0) {
    if (m < hoge[index].count) {
      money += m * hoge[index].price;
      m = 0;
    } else {
      money += hoge[index].count * hoge[index].price;
      m -= hoge[index].count;
    }
    ++index;
  }

  cout << money << endl;

  return 0;
}