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 drink{
  long price;
  long stock;
}drink;

int compare(const void *a,const void *b){
  drink *A =(drink *)a;
  drink *B =(drink *)b;
  return A->price - B->price;
}

int main(){
  int N,M;
  scanf("%d%d",&N,&M);
  drink* D =(drink*)malloc(sizeof(drink)*N);
  for(int i=0;i<N;i++){
    scanf("%d%d",&D[i].price,&D[i].stock);
  }
  qsort(D,N,sizeof(drink),compare);

  long cost = 0;
  long item = 0;
  for(int j=0;j<N;j++){
    if(item + D[j].stock < M){
      cost += (D[j].price)*(D[j].stock);
      item += D[j].stock;
    }else if(item + D[j].stock >= M){
      cost += (D[j].price)*(M - item);
      item += M;
      j = N+1;
    }
  }
  printf("%ld\n",cost);
  
  return 0;
}