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: Wrong Answer)

#include<stdio.h>
#include<stdlib.h>
typedef struct{
  long long int value;
  int cost;
} box;
int cmp( const void *p, const void *q ) {
    return ((box*)p)->cost - ((box*)q)->cost;
}

int main(void) {
  int a,b;
  long long wallet=0;
  scanf("%d %d",&a,&b);
  box store[a];
  for(int i=0;i<a;i++){
    scanf("%d %d",&(store[i].cost),&(store[i].value));
}
  qsort(store,a,sizeof(box),cmp);
 
  int num=0,want=0;
  while(b-want>0){
      if(store[num].value>0){
	wallet+=store[num].cost;
	store[num].value--;
	want++;}
      else num++;
      
}
  printf("%d",wallet);
  return 0;
}