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>
#define ll long long

typedef struct{
  ll A;
  int B;
}energy;
int cmp(const void *p,const void *q){
  return ((energy*)p)->A-((energy*)q)->A;
}
void init(energy *t,int N);

int main(){
  int N,M;
  energy *E;
  ll ans=0;
  int i;

  scanf("%d%d",&N,&M);
  E=(energy*)malloc(sizeof(energy)*N);

  init(E,N);
  qsort(E,N,sizeof(energy),cmp);

  for(i=0;i<N;i++){
      if(E[i].B<=M){
        ans+=E[i].A*E[i].B;
        M-=E[i].B;
      }else{
        ans+=M*E[i].A;
        break;
      }
  }
  printf("%lld\n",ans);

  free(E);
  return 0;
}

void init(energy *t,int N){
  for(int i=0;i<N;i++)scanf("%lld%lld",&t[i].A,&t[i].B);
}