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: Time Limit Exceeded)

init = input().split(' ')
init = [int(i) for i in init]
n,m= init[0],init[1]

ab = []

try:
	while True:
		inp1 = input().split(' ')
		inp1 = [int(i) for i in inp1]
		ab.append([inp1[0],inp1[1]])
except:
	pass

ab.sort()
total = sum([ab[j][0]*ab[j][1] for j in range(n)])
deltarget = sum([ab[j][1] for j in range(n)]) - m

while deltarget > 0:
	if ab[len(ab)-1][1]>0:
		total -= ab[len(ab)-1][0]
		ab[len(ab)-1][1] -= 1
		deltarget -= 1
		
	else:
		ab.pop(len(ab)-1)

print(total)