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: Accepted)

#include<stdio.h>
#include<stdint.h>

//入れ替え
void swap(uint64_t a[],int i,int j){
	int t=a[i];
	a[i]=a[j];
	a[j]=t;
}

//昇順クイックソート
void Qsort(uint64_t A[],uint64_t B[],int left,int right){
	int i=left,j=right,p=A[(left+right)/2];
	
	while(1){
		while(A[i]<p) i++;
		while(p<A[j]) j--;
		if(i>=j) break;
		swap(A,i,j);
		swap(B,i,j);
		i++;
		j--;
	}

	if(left<i-1) Qsort(A,B,left,i-1);
	if(j+1<right) Qsort(A,B,j+1,right);
}

int main(void){
	uint64_t ans=0;
	int N,M,i,count=0;
	scanf("%d %d",&N,&M);
	uint64_t A[N],B[N];
	for(i=0;i<N;i++) scanf("%llu %llu",&A[i],&B[i]);
	
	Qsort(A,B,0,N-1);
	
	for(i=0;i<N;i++){
		if(count+B[i]>=M){
			ans+=A[i]*(M-count);
			break;
		}
		else{
			ans+=(uint64_t)A[i]*B[i];
			count+=B[i];
			//printf("ans=%llu,count=%d\n",ans,count);
		}
	}
	
	printf("%llu",ans);
}