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 A;
  int B;
}Store;

Store S[100000];

int Cmp(const void *a, const void *b){
  return ((Store *)a)->A - ((Store *)b)->A;
}

void Do(int N, int M){
  int i, m;
  long long n;
  for(i = 0;i < N;i++) scanf("%d %d", &S[i].A, &S[i].B);
  qsort(S, N, sizeof(Store), Cmp);
  m = 0;
  n = 0;
  for(i = 0;i < N;i++){
    if(m + S[i].B < M){
      n += (long long) S[i].A * S[i].B;
      m += S[i].B;
    }else{
      n += (long long) S[i].A * (M - m);
      break;
    }
  }
  printf("%lld\n", n);
}

int main(void){
  int N, M;
  scanf("%d %d", &N, &M);
  Do(N, M);
  return 0;
}