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>
#define rep(i,cc,n) for(int i=cc;i<=n;++i)
using namespace std;
using ll = long long;

int main() {
  ll N, M;
  cin >> N >> M;

  map<ll,ll> m;
  rep(i,0,N-1) {
    ll k, v;
    cin >> k >> v;
    m[k] = v;
  }

  ll ans = 0, cnt = 0;
  for (pair<ll,ll> p : m) {
    if (cnt + p.second < M) {
      ans += p.first * p.second;
      cnt += p.second;
    } else {
      while (0 < p.second && cnt < M) {
        ans += p.first;
        p.second--;
        cnt++;
      }
    }
  }

  cout << ans << endl;
  return 0;
}