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 Pascal (FPC 2.6.2) (judge verdict: Accepted)

var
	a,b:array[0..100010] of longint;
	ans:int64;
	n,m,i:longint;
procedure qsort(l,r:longint);
var
	i,j,t,mid:longint;
begin
	i:=l; j:=r; mid:=a[(l+r) div 2];
	repeat
		while a[i]<mid do inc(i);
		while a[j]>mid do dec(j);
		if i<=j then
		begin
			t:=a[i]; a[i]:=a[j]; a[j]:=t;
			t:=b[i]; b[i]:=b[j]; b[j]:=t;
			inc(i); dec(j);
		end;
	until i>j;
	if l<j then qsort(l,j);
	if i<r then qsort(i,r);
end;
begin
	readln(n,m);
	for i:=1 to n do read(a[i],b[i]);
	qsort(1,n);
	ans:=0; i:=1;
	while m>0 do
	begin
		if m<=b[i] then
		begin
			ans:=ans+m*a[i];
			break;
		end;
		m:=m-b[i];
		ans:=ans+a[i]*b[i];
		inc(i);
	end;
	writeln(ans);
end.