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 lli long long int
using namespace std;

int main()
{
	lli n, m, ai, bi;
	
	cin >> n >> m;
	
	vector< pair<lli, lli> > v;
	
	lli sum = 0, mn;
	
	for(int i = 0; i < n; i++){
		cin >> ai >> bi;
		
		v.push_back(make_pair(ai, bi));
	}
	
	sort(v.begin(), v.end());
	
	for(int i = 0; i < n; i++){
		if(m == 0) break;
		
		ai = v[i].first; bi = v[i].second;
		
		mn = min(m, bi);
		sum += mn*ai;
		
		m -= mn;
	}
	
	cout << sum << "\n";
	
	return 0;
}