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: Accepted)

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

typedef struct {
    int zaiko;             //zaiko
    int price;          //価格
} cell;

// --------------- 比較用の関数 cmp -------------------
int cmp( const void *p, const void *q ) {
    return ((cell*)p)->price - ((cell*)q)->price;
}

int main(){
  long int max=0;
  
  int N,M,i;
  scanf("%d %d",&N,&M);
  cell A[N];
  for(i=0;i<N;i++){
    scanf("%d %d",&A[i].price,&A[i].zaiko);
  }
   int n = sizeof A / sizeof( cell );
    qsort( A, n, sizeof(cell), cmp );
  i=0;
  while(M!=0){
    if(A[i].zaiko <= M){
      M-=A[i].zaiko;
      max += (long)A[i].price * (long)A[i].zaiko;
      i++;
    }
    else{
      max += (long)M * (long)A[i].price;
      M=0;
    }
  }
  
  printf("%ld\n",max);
  return 0;
}