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: Accepted)

#include <iostream>
#include <vector>
#include <algorithm>


using namespace std;


using ll = long long;


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

    for (ll i = 0; i < N; i++) {
        ll a, b;
        cin >> a >> b;
        shops.push_back(make_pair(a, b));
    }

    sort(shops.begin(), shops.end(), [](auto &l, auto &r) { return l.first < r.first; });

    ll now = 0;
    ll ans = 0;
    for (auto it = shops.begin(); it != shops.end() && now < M; ++it) {
        if (now + it->second >= M) {
            ans += (M - now) * it->first;
            now += (M - now);
        } else {
            ans += it->first * it->second;
            now += it->second;
        }
    }

    cout << ans << endl;
}