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)

N, M = map(int, input().split(' '))
A = []
B = []
for n in range(N):
  A_n, B_n = map(int, input().split(' '))
  A.append(A_n)
  B.append(B_n)
  
# calc answer
drink = 0
ans = 0
while(drink < M):
  min_i = A.index(min(A))  # min shop
  if B[min_i] <= M-drink:  # buy all drinks
    drink += B[min_i]
    ans += A[min_i]*B[min_i]
    del A[min_i]
    del B[min_i]
  else:  # buy some drinks
    ans += A[min_i]*(M-drink)
    drink = M
  
print(ans)