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 <iostream>
#include <vector> 
#include <algorithm>
#include <functional>
using namespace std;

struct price{
    int money;
    int num;
};

int main(){
	std::vector<price> prices;
	int m,mon,x,y,ans =0;
	cin >> m >> mon;
	int remein_m = mon;
    for(int i = 0; i < m; i ++){
        cin >> x >> y;
        prices.push_back({x,y});
    }
    sort(prices.begin(), prices.end(), [](const price& x, const price& y) { return x.money < y.money;});
    
    for(int i = 0; i < m ; i ++){
        cout << remein_m <<"|"  << ans << endl;
        if(prices[i].num >= remein_m){
            ans += remein_m*prices[i].money;
            break;
        }else{
            ans += prices[i].num*prices[i].money;
            remein_m -=prices[i].num;
        }
    }
    cout << ans << endl;
	return 0;
}