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: Runtime Error)

#include<iostream>
#define p 10000
using namespace std;

void swap(long int *x,long int *y){
	long int temp=*x;
	*x=*y;
	*y=temp;
}

void swapl(long long int *x,long long int *y){
	long long int temp=*x;
	*x=*y;
	*y=temp;
}

int main(){
	long int N,M;
	long long int A[p];
	long int B[p];
	int i,j;
	long long int sum=0;
	long long int min=0;
	int min_i;
	int x=0;
	
	cin >> N >> M;
	
	for(i=0;i<N;i++){
		cin >> A[i] >> B[i];
	}
	
	for(j=0;j<N;j++){
		min=A[j];
		min_i=j;
		for(i=j+1;i<N;i++){
			if(A[i]<min){
				min=A[i];
				min_i=i;
			}
		}
		swapl(&A[j],&A[min_i]);
		swap(&B[j],&B[min_i]);
	}
	
	for(i=0;i<N;i++){	
		for(j=0;j<B[i];j++){
			sum+=A[i];
			x++;
			if(x==M){
				cout << sum << endl;
				break;
			}
		}
	}
	
	return 0;
}