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 pair{
  long a;
  int b;
};

int compare(const void *x, const void *y) {
  return ((struct pair *)x)->a - ((struct pair *)y)->a;
}

int main() {
  int N,M;
  scanf("%d %d",&N,&M);
  struct pair AB[N];
  for (int i = 0; i < N; i++) {
    scanf("%ld %d",&AB[i].a,&AB[i].b);
  }
  qsort(AB,N,sizeof(struct pair),compare);

  struct pair ab = {0,M};
  for (int i = 0; i < N; i++) {
    if (ab.b >= AB[i].b) {
      ab.a += AB[i].a*AB[i].b;
      ab.b -= AB[i].b;
    }
    else {
      ab.a += AB[i].a*ab.b;
      break;
    }
  }
  printf("%ld\n",ab.a);
  return 0;
}