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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <time.h>

#define TLong long long

typedef struct {
	TLong a;	// price
	TLong b;	// limit
} store;

int comp(const void *p,const void *q){
    return ((store *)p)->a - ((store*)q)->a;
}

int main(int argc, char const *argv[])
{
	TLong n,m;
	TLong t_price = 0;
	TLong temp;
	store st[100000];
	scanf("%lld%lld",&n,&m);
	for (TLong i = 0; i < n; ++i)	scanf(" %lld%lld", &st[i].a, &st[i].b);

	qsort(st,n,sizeof(store),comp);
	for (int i = 0; (i < n && m > 0); ++i)
	{
		temp = (st[i].b <= m) ? st[i].b : m;
		t_price += st[i].a * temp;
		m -= temp;
	}
	printf("%lld\n",t_price);
	return 0;
}