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 C (GCC 5.4.1) (judge verdict: Time Limit Exceeded)

main()
{
    long long N, M, A[100000], B[100000], i, c=1000000001,idx=0, get=0, ans=0, g=0;
    scanf("%lld%lld",&N,&M);
    for(i=0;i<N;i++)scanf("%lld%lld", &A[i], &B[i]);
    do
    {
        c=1000000001;
        for(i=0;i<N;i++)
        {
            if(A[i]<c){c=A[i];idx=i;}
        }
        g=B[idx]<=M-get?B[idx]:M-get;
        ans+=A[idx]*g;get+=g;
        A[idx]=1000000001;
    }while(get<M);
    printf("%lld",ans);
}