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 <stdint.h>
#include <stdlib.h>
#include <inttypes.h>

#define SIZE (10*10*10*10*10)

typedef struct _ab {
  int a;
  int b;
} ab_t;

int compare(ab_t *a, ab_t *b)
{
  return a->a - b->a;
}

void bsort(ab_t *ab, int size)
{
  int i;
  int change = 1;
  ab_t tmp;

  while (change) {
    change = 0;
    for(i=0; i<(size - 1); i++) {
      if (ab[i].a > ab[i+1].a) {
        tmp = ab[i];
        ab[i] = ab[i+1];
        ab[i+1] = tmp;
        change = 1;
      }
    }// for
  }//while

  return;
}

int main()
{
  int cnt;
  int  N, M;
  long long int  A[SIZE] = {0,};
  long long int  B[SIZE] = {0,};
  ab_t ab[SIZE] = {{0,0},};
  long long int cost;
  long long int num;
  int i,j;
  int result = 0;

  cnt = scanf("%d %d", &N, &M);

  for (i=0; i<N; i++) {
    cnt = scanf("%lld %lld", &A[i], &B[i]);
    ab[i].a = A[i];
    ab[i].b = B[i];
  }

  //bsort(&ab, N);
  qsort(ab, N, sizeof(ab_t), compare);

  cost = 0;
  result = 0;
  for (i=0; i<N; i++) {
    if (result < M) {
      num = M - result;
      if (num > ab[i].b) {
        num = ab[i].b;
      }
      cost += num * ab[i].a;
      result += num;
    } else {
      break;
    }
  }

  printf("%llu", (unsigned long long int)cost);

  return 0;
}