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>
#define rp(i,n) for(int i=0; i<n; i++)
using namespace std;
using ll=long long;
using P=pair<ll,ll>;

int main(){
    ll n,m;
    cin >> n >> m;
    vector<P> ab(n);
    rp(i,n){
        ll a,b;
        cin >> a >> b;
        ab.at(i)=make_pair(a,b);
    }
    sort(ab.begin(),ab.end());
    int ctr=0;
    ll money=0ll;
    while(m>0){
        if(m<ab.at(ctr).second){
            money += ab.at(ctr).first*m;
            m=0;
        }
        else{
            money += ab.at(ctr).first*ab.at(ctr).second;
            m-=ab.at(ctr).second;
            ctr++;
        }
    }
    cout << money << endl;
    return 0;
}