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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include <stdio.h>
void swap (long long int *x,long long int *y) {
  long long temp = *x;
  *x = *y;
  *y = temp;
}
int partition (long long int array[],long long int ary[],long long int left,long long int right) {
  long long int i, j, pivot;
  i = left;
  j = right + 1;
  pivot = left;
  do {
    do { i++; } while (array[i] < array[pivot]);
    do { j--; } while (array[pivot] < array[j]);
    if (i < j) {
      swap(&array[i], &array[j]);
      swap(&ary[i], &ary[j]);
    }
  } while (i < j);
  swap(&array[pivot], &array[j]);
  swap(&ary[pivot], &ary[j]);
  return j;
}
void quick_sort (long long int array[],long long int ary[],long long int left,long long int right) {
  long long int pivot;
  if (left < right) {
    pivot = partition(array,ary,left,right);
    quick_sort(array,ary,left,pivot-1);
    quick_sort(array,ary,pivot+1,right);
  }
}
int main(){
  long long int n,m,cnt = 0,ans = 0;
  scanf("%lld %lld",&n,&m);
  long long int a[n],b[n];
  for(int i = 0;i < m;i++){
    scanf("%lld %lld",&a[i],&b[i]);
  }
  quick_sort(a,b,0,n - 1);
  for(int i = 0;i < n;i++){
    if(b[i] < m){
      m -= b[i];
      ans += a[i] * b[i];
    }else{
      printf("%lld\n",ans + a[i] * m);
      return 0;
    }
  }
}