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>
#include<math.h>
 
#include <stdlib.h>
 
int compare_int(const void *a, const void *b)
{
    return *(int*)a - *(int*)b;
}
 
 
int main(void)
{
  int i,t,p;
  long long x,y,z=0,k=0;
  long long a[2][100000],b[100000];
  
  scanf("%ll %ll",&x,&y);
  
  for(i=0;i<x;i++)
  {
    scanf("%ll %ll",&a[0][i],&b[i]);
    a[1][i]=i;
  }
 
  qsort(a[0], x, sizeof(long long), compare_int);
  
for(t=0;t<x;t++)
{
  p=0;
    i=a[1][t];
  while(z<y&&p<i)
  {
    k=k+a[0][t];
    p++;
    z++;
  }
 
  if(z<y)
  {
    break;
  }
}
  
  printf("%ll",k);
  
  return 0;
  
}