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 Ruby (2.3.3) (judge verdict: Runtime Error)

x = input().split(" ")
store = int(x[0])
need = int(x[1])
ans = 0
price = {}

for i in range(store):
    y = input().split(" ")
    pri = int(y[0])
    bottles = int(y[1])
    if pri in price.keys():
        price[pri] += bottles
    else:
        price[pri] = bottles
#print(price)
price_sort = sorted(price.items(), key=lambda x: x[0]) #安い順に並び変え
#print(price_sort)
for price_bottles in price_sort: #price_bottles=(価格、本数)になる
    if need == price_bottles[1]:
        ans += price_bottles[1] * int(price_bottles[0])
        break
    elif need > price_bottles[1]:
        ans += price_bottles[1] * int(price_bottles[0])
        need -= price_bottles[1]
    else:
        ans += need * int(price_bottles[0])
        break


print(ans)