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>
typedef struct {
  int a;
  int b;
} Drink;
int compare_int(const void *a,const void *b){
  return *(int *)a-*(int *)b;
}
int main(void)
{
  int n,m;
  long long ans=0;
  Drink array[100010];
  scanf("%d%d",&n,&m);
  for(int i=0;i<n;i++){
    scanf("%d%d",&array[i].a,&array[i].b);
  }
  qsort(array,n,sizeof(Drink),compare_int);

  /*for(int i=0;i<n;i++){
  *  printf("%d %d\n",array[i].a,array[i].b);
  *}
  */

  for(int i=0;m>0;i++){
    while((array[i].a>0)&&(m>0&&array[i].b>0)){
      ans+=array[i].a;
      array[i].b--;
      m--;
    }
  }
  printf("%lld",ans);

  return 0;
}