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 <math.h>
#include <string.h>
#include <ctype.h>

typedef struct {	/* 構造体の宣言 */
	long a;
	long b;
} drink;

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

int main(void) {
  long n, m;
  long ans = 0, kari = 0, k = 0;
	drink lst[100002];

  if(scanf("%ld %ld", &n, &m) == 1);

  for(int i = 0; i < n; i++) {
    if(scanf("%ld %ld", &lst[i].a, &lst[i].b) == 1);
  }

	long kazu = sizeof lst / sizeof( drink );

	qsort(lst, n, sizeof(drink), quick);

  kari = 0;
  while(kari <= m) {
    ans = ans + lst[k].a * lst[k].b;
    kari = kari + lst[k].b;
    k++;
  }

  k--;
  if(kari != m) {
		ans = ans - (lst[k].a * (kari - m));
  }



  printf("%ld", ans);

  return 0;
}