Score : 300 points
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 is given from Standard Input in the following format:
N M A_1 B_1 A_2 B_2 \vdots A_N B_N
Print the minimum amount of money with which Takahashi can buy M cans of energy drinks.
2 5 4 9 2 4
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.
4 30 6 18 2 5 3 10 7 9
130
1 100000 1000000000 100000
100000000000000
The output may not fit into a 32-bit integer type.