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-----
2 5
4 9
2 4

-----Sample Output-----
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.

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