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: Wrong Answer)

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

int main() {
  int N,M;
  cin>>N>>M;
  vector<pair<int,int>>a;

  for (int i = 0; i < N; i++) {
    int A,B;
    cin >> A>>B;
    a.push_back(pair<int,int>(A,B));
  }
  sort(a.begin(), a.end());
  long ans=0;
  
    for (int i = 0; i < N; i++) {
      if(a.at(i).second<=M){
        long b=a.at(i).first;
        ans+=b*a.at(i).second;
        M-=a.at(i).second;
      }
      else{
        ans+=a.at(i).first*M;
        break;
      }
    }
  
  
    cout<<ans<<endl;
  
}