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 drink {
  int cost;
  int num;
};

int asc(const void *a, const void *b) {
  return (*(struct drink*)a).cost - (*(struct drink*)b).cost;
}

int main() {
  int n, m, i = 0, count = 0, ncount = 0;
  long int price = 0;
  scanf("%d%d", &n, &m);
  struct drink st[n];
  for (size_t i = 0; i < n; i++) {
    scanf("%d%d", &(st[i].cost), &(st[i].num));
  }
  qsort(st, n, sizeof(struct drink), asc);
  while (count < m) {
    price += st[i].cost;
    if (ncount+1 >= st[i].num) {
      i++;
      ncount = 0;
    } else {
      ncount++;
    }
    count++;
  }
  printf("%ld\n", price);
  return 0;
}