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 <stdlib.h>
#include <stdint.h> // uint64_t

#define max(a,b) ((a) > (b) ? (a) : (b))
#define min(a,b) ((a) > (b) ? (b) : (a))

int get_int2(int *a1, int *a2) {
  scanf("%d %d", a1, a2);
  return 0;
}

struct drink {
    int price;
    int stock;
};

int asc_by_price(const void *a1, const void *a2) {
    struct drink *d1 = (struct drink*)a1;
    struct drink *d2 = (struct drink*)a2;
    return d1->price - d2->price;
}

#define NUM_MAX 100000

int main(void) {
    int num, llimit;
    static struct drink drinks[NUM_MAX];
    get_int2(&num, &llimit);
    int i;
    for(i = 0; i < num; i++) {
        get_int2(&drinks[i].price, &drinks[i].stock);
    }
    qsort(drinks, num, sizeof(struct drink), asc_by_price);

    int64_t cost = 0;
    for(i = 0; i < num; i++) {
        if(llimit <= 0) break;
        int n = min(llimit, drinks[i].stock);
        cost += (int64_t)drinks[i].price * n;
        llimit -= n;
    }
    printf("%lld\n", cost);
    return 0;
}