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 <string.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <limits.h>
#include <math.h>
int cmp_char(const void *a, const void *b) { return *(char *)a - *(char *)b; }                     /* a,b,c,d.. */
int cmp_str(const void *a, const void *b) { return strcmp(*(const char **)a, *(const char **)b); } /* aaa,aab.. */
#define min(a, b) (a < b ? a : b)
#define max(a, b) (a > b ? a : b)
#define read(n) scanf("%d", &n)
#define readl(n) scanf("%lld", &n)
#define rep(i, l, r) for (int i = l; i < r; i++)
#define MAX 100001
#define MOD 1000000007

typedef long long int lli;

lli a[MAX], b[MAX];

int acs(const void *x, const void *y)
{
  if (a[*(int *)x] - a[*(int *)y] > 0)
    return 1;
  if (a[*(int *)x] - a[*(int *)y] < 0)
    return -1;
  return 0;
}

int main(void)
{
  int n, m;
  read(n);
  read(m);

  rep(i, 0, n)
  {
    readl(a[i]);
    readl(b[i]);
  }

  int idx[MAX];
  rep(i, 0, n) idx[i] = i;

  qsort(idx, n, sizeof(int), acs);

  int rem = m;
  lli mny = 0;
  rep(i, 0, n)
  {
    if (rem <= 0)
      break;
    int drink = min(rem, b[idx[i]]);
    mny += a[idx[i]] * drink;
    rem -= drink;
  }

  printf("%lld\n", mny);
  return 0;
}