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>

struct PRICE{
  long long int A;
  long long int B;
};

int cmp( const void *p, const void *q ) {
    return ((struct PRICE*)p)->A - ((struct PRICE*)q)->A;
}

int main(void){
  int N,M;
  scanf("%d %d", &N, &M);
  struct PRICE* price = (struct PRICE*)malloc(sizeof(struct PRICE) * N);
  for(int i = 0;  i < N; i++){
    scanf("%lld %lld", &price[i].A, &price[i].B);
    //printf("%d %d\n", price[i].A, price[i].B);
  }
  qsort(price, N, sizeof(struct PRICE), cmp);
  int M_cnt = M;
  long long int sum = 0;
  for(int i = 0;  i < N; i++){
    M_cnt -= price[i].B;
    if(M_cnt <= 0){
      sum += price[i].A * (price[i].B + M_cnt);
      break;
    }
    sum += price[i].A * price[i].B;
    //printf("%d %d\n", price[i].A, price[i].B);
  }
  printf("%lld", sum);  
  return 0;
}