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>
int a[100005], b[100005];
int asc(const void *x, const void *y)
{
    return a[*(int *)x] - a[*(int *)y];
}
int main()
{
    int n, m;
    scanf("%d %d", &n, &m);
    int i;
    for (i = 0; i < n; i++)
        scanf("%d %d", &a[i], &b[i]);
    int c[100005];
    for (i = 0; i < n; i++)
        c[i] = i;
    qsort(c, n, sizeof(int), asc);
    long long int ans = 0;
    for (i = 0;; i++)
    {
        if (m - b[c[i]] > 0)
        {
            m -= b[c[i]];
            ans += ((long long int)a[c[i]]) * ((long long int)b[c[i]]);
        }
        else
        {
            ans += ((long long int) m) *((long long int) a[c[i]]);
            break;
        }
    }
    printf("%lld\n", ans);
    return 0;
}