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>

int
main()
{
  int N,M;
  int A[N],B[N],num_min[N];
  int i,j;
  int num_min_b;
  int mon=0;
  int min[N];
  
  for(i=0;i<N;i++){
    scanf("%d %d",A[i],B[i]);
  }
  
  for(i=0;i<N;i++){
    min[i]=A[0];
    for(j=0;j<N;j++){
    	if(min[i]>A[j]){
      	min[i] = A[j];
        num_min[i]=j;
        }
    }
     A[num_min[i]]=1000000000+1;
  }
  
  int cnt=0;
  int goukei=0;
  for(i=0;i<N;i++){
	for(j=0;j<B[num_min[i]];j++){
      	mon += min[i];
      	goukei++;
    	if(goukei==M)  goto OUT;
    }
  }
  OUT:
  	printf("%d",mon);
  
  	return 0;
  
}