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.

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.

Constraints

• All values in input are integers.
• $1 \leq N, M \leq 10^5$
• $1 \leq A_i \leq 10^9$
• $1 \leq B_i \leq 10^5$
• $B_1 + ... + B_N \geq M$

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.

见题面 Input 部分

见题面 Output 部分