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>
#define max(X, Y) ((X) > (Y) ? (X) : (Y))
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
typedef long long ll;

typedef struct {
	ll cost, cnt;
} Item;

ll n, m;
Item a[100005];

int cmp(const Item *a, const Item *b);

int main()
{
	ll ans = 0;
	ll drink = 0;

	scanf("%lld %lld", &n, &m);
	for (ll i = 0; i < n; i++) {
		scanf("%lld %lld", &a[i].cost, &a[i].cnt);
	}

	qsort(a, n, sizeof(Item), (int (*)(const void *, const void *))cmp);
	
	ll i = 0;
	while (drink < m) {
		if (drink + a[i].cnt <= m) {
			ans = ans + a[i].cost * a[i].cnt;
			drink = drink + a[i].cnt;
		} else {
			ll work = m - drink;
			ans = ans + a[i].cost * work;
			drink += work;
		}

		i++;
	}

	printf("%lld\n", ans);

	return 0;
}

int cmp(const Item *a, const Item *b)
{
	if (a->cost > b->cost) {
		return 1;
	} else if (a->cost == b->cost) {
		return 0;
	} else {
		return -1;
	}
}