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: Wrong Answer)

#include <stdio.h>
#include <string.h>
 
int main(void){
  int n,m;
  int i;
  int kei = 0;
  int a[100000];
  int b[100000];
  scanf("%d %d",&n,&m);
  for (i = 0; i<n; i++){
     scanf("%d %d", &a[i],&b[i]);  //ここまで配列の準備
  }
  int tmp,j,tmp_b;
  i = 0;
  for (i = 1; i<n; i++) {
    tmp = a[i];
    tmp_b = b[i];
    if (a[i - 1] > tmp) {
        j = i;
        do {
            a[j] = a[j - 1];
            b[j] = b[j - 1];
            j--;
        } while (j > 0 && a[j - 1] > tmp);
        a[j] = tmp;
        b[j] = tmp_b;
    }
  }
  for (i = 0; i<n; i++){
    if(m>b[i]){
      kei = (a[i]*b[i]) + kei;
    }else{
      kei = kei + (a[i]*m);
      break;
    }
    m = m - b[i];
  }
  printf("%d",kei);
  return 0;
}