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 int long long
#define ll long long
#define rep(i,n) for(int i=0;i<n;i++)
#define INF LLONG_MAX/3
#define eps LDBL_EPSILON
#define moder 1000000007
#define pie 3.141592653589793238462643383279
#define P std::pair<int,int>
#define prique priority_queue
using namespace std;
int n,m;
P a[100010];
signed main(){
	cin>>n>>m;
	rep(i,n)cin>>a[i].first>>a[i].second;
	sort(a,a+n);
	int ans=0,now=0;
	rep(i,n){
		if(now+a[i].second<=m){
			ans+=a[i].first*a[i].second;
			now+=a[i].second;
		}
		else{
			ans+=(m-now)*a[i].first;
			cout<<ans<<endl;
			return 0;
		}
	}
	cout<<ans<<endl;
	return 0;
}