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: Time Limit Exceeded)

#include <stdio.h>
#include <stdlib.h>


long A[100000],B[100000];

void q_sort(int left, int right){
    long pivot_A,pivot_B, l_hold, r_hold;

    l_hold = left;
    r_hold = right;
    pivot_A = A[left];
    pivot_B = B[left];
    while (left < right)
    {
        while ((A[right] >= pivot_A) && (left < right))
            right--;
        if (left != right)
        {
            A[left] = A[right];
            B[left] = B[right];
            left++;
        }
        while ((A[left] <= pivot_A) && (left < right))
            left++;
        if (left != right)
        {
            A[right] = A[left];
            B[right] = B[left];
            right--;
        }
    }
    A[left] = pivot_A;
    B[left] = pivot_B;
    pivot_A = left;
    pivot_B = left;
    left = l_hold;
    right = r_hold;
    if (left < pivot_A)
        q_sort(left, pivot_A-1);
    if (right > pivot_A)
        q_sort(pivot_A+1, right);
}

int main(){
    int N,M,i,j,count=0;
    long sum=0;

    scanf("%d %d", &N,&M);
    for(i=0;i<N;i++){
      scanf("%ld %ld", &A[i],&B[i]);
    }

    q_sort(0,N-1);

    for(i=0;i<N;i++){
      for(j=0;j<B[i];j++){
        sum=sum+A[i];
        count++;
        if(count==M){
          printf("%ld\n", sum);
          return 0;
        }
      }
    }
}