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 Python (3.4.3) (judge verdict: Wrong Answer)

class Shop():
    def __init__(self, a, b):
        self.a = a
        self.b = b

N, M = [ int(x) for x in input().split(' ')]
shops = []
for i in range(N):
    a, b = [int(x) for x in input().split(' ')]
    shops.append(Shop(a, b))

shops.sort(key=lambda x: x.a)

cost = 0
bought = 0
for i in range(N):
    shop = shops[i]
    print(i, cost, bought, shop.a, shop.b)
    if (shop.b <= M - bought):
        cost += shop.a * shop.b
        bought += shop.b
    else:
        cost += shop.a * (M - bought)
        bought += (M - bought)
    if (bought == M):
        break

print(cost)