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 <bits/stdc++.h>

using namespace std;
using ll = long long int;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#define rep(i, N) for (ll i = 0; i < (N); i++)
#define FOR(i, a, b) for (ll i = (a), i < (N): i++)
// #define all(A) A.begin(),A.end()

int main(){
    int N, M;
    cin >> N >> M;
    vector<pll> AB(N);
    rep(i, N) {
        cin >> AB[i].first >> AB[i].second; 
    }
    sort(AB.begin(), AB.end());
    ll ans = 0;
    int i = 0;
    while (true) {
        if (AB[i].second >= M) {
            ans += M * AB[i].first;
            cout << ans << endl;
            return 0;
        }
        else {
            M -= AB[i].second;
            ans += AB[i].first * AB[i].second;
            i++;
        }

    }

    return 0;
}