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: Time Limit Exceeded)

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

int main(){
  int i,j,k,honsu;
  long long int N,M,ans=0,tmp=0,A[10000000],B[10000000];
  scanf("%lld %lld",&N,&M);
  //*A = (long *)malloc(sizeof(long)*(N+1));
  //*B = (long *)malloc(sizeof(long)*(N+1));
  for(i=0;i<N;i++){
    scanf("%lld %lld",&A[i],&B[i]);
  }
  honsu=0;ans=0;
  for (i = 0; i<(N); i++) {
    for (j = (N - 1); j > i; j--) {
      if (A[j-1] > A[j]) {
        tmp = A[j-1];
        A[j-1] = A[j];
        A[j] = tmp;
        tmp = B[j-1];
        B[j-1] = B[j];
        B[j] = tmp;
      }
    }
    for(k=0;k<B[i];k++){
      ans+=A[i];  
      honsu++;
      if(honsu>=M){
        printf("%lld\n",ans);
        return 0;
      }
    }
  }

}