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: Accepted)

#include <stdio.h>
#include <stdlib.h>
int compare_int(const void *a, const void *b)
{
  return *(int*)a - *(int*)b;
}
int main(void){
  long i;
  long len,count;
  long price=0;
  long ii[1000000][2];
  scanf("%ld %ld",&len,&count);
  for(i=0;i<len;i++){
    scanf("%ld %ld",&ii[i][0],&ii[i][1]);
  }
  qsort(ii, len, sizeof(ii[0]), compare_int);
  for(i=0;i<len;i++){
//       printf("i%ld %ld %ld\n",i,ii[i][0],ii[i][1]);
  }
  for(i=0;i<len;i++){
        if(ii[i][1] >= count){
			price += ii[i][0] * count;
			printf("%ld\n",price);
          	return 0;
        }
    	else{
			price += ii[i][1] * ii[i][0];
			count -= ii[i][1];
        }
  }
  return 0;

}