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_2=0
# B_2=0
list_A=[]
list_B=[]
sum_num=0
for i in range(n):
    A,B = map(int,input().split())
    list_A.append(A)
    list_B.append(B)

for i in range(n):
    minj = i
    for j in range(i,n):
        if list_A[j]<list_A[minj]:
            minj=j
    hokan=list_A[i]
    list_A[i]=list_A[minj]
    list_A[minj]=hokan
    hokan_b=list_B[i]
    list_B[i]=list_B[minj]
    list_B[minj]=hokan_b

list_AB = []
for i in range(n):
    list_AB.append( (list_A[i], list_B[i]))

list_AB.sort()
list_A , list_B = zip(*list_AB)
# list_A.sort()
# list_B.sort()
# print(list_A)
# print(list_B)


m_1=m
for i in range(n):
    m_2=m_1
    m_1=m_1-int(list_B[i])
    if m_1<0:
        sum_num=sum_num + int(list_A[i])*m_2
    else:
        sum_num=sum_num +int(list_A[i])*int(list_B[i])
    #print(sum_num)
    #print("m_1",m_1,"m_2",m_2)
    if m_1<0:
        break

print(sum_num)