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: Runtime Error)

#include<stdio.h>
int main(void){
  int n, m, a[1000], b[1000], i, j, tmp, yasui, sum = 0, yobun, k, goukei = 0;

  scanf("%d %d", &n, &m);
  for(i = 0; i < n; i++) scanf("%d %d", &a[i], &b[i]);
  for(i = 0; i < n - 1; i++){
    yasui = i;
    for(j = i + 1;j < n; j++) if(a[j] < a[yasui]) yasui = j;
    tmp = a[i]; a[i] = a[yasui]; a[yasui] = tmp;
    tmp = b[i]; b[i] = b[yasui]; b[yasui] = tmp;
  }
  for(k = 0; sum < m; k++){
    sum = sum + b[k];
    if (sum > m){
      yobun = sum - m;
      b[k] = b[k] - yobun;
    }
    goukei = goukei + a[k] * b[k];
  }
  printf("%d\n", goukei);
  return 0;
}