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 (GCC 5.4.1) (judge verdict: Runtime Error)

#include <bits/stdc++.h>
using namespace std;

struct shop {
  long long cost;
  long long amount;
  bool operator<( const shop& right ) const {
    if(cost != right.cost)
      return cost < right.cost;
  }
};

int main() {
  int N, M;
  long long ans = 0;
  cin >> N >> M;
  vector<shop> shops(N);
  for(int i = 0; i < N; i++){
    cin >> shops[i].cost >> shops[i].amount;
  }
  sort(shops.begin(), shops.end());
  
  int bought = 0, idx = 0;

  while(bought < M){
    if(bought == M) break;
    if(shops[idx].amount <= (M - bought)){
      ans += (long long)shops[idx].cost * shops[idx].amount;
      bought += shops[idx].amount;
    }else{
      ans += shops[idx].cost * (M - bought);
      bought += (M - bought);
    }
    idx++;
  }

  cout << ans << endl;
}