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: Runtime Error)

#include<stdio.h>

int main(){
	long int n,m;
	scanf("%ld %ld",&n,&m);
	long long int sum = 0;
	long long int buy = 0;
	long long int store[n][2];
	long long int visit[n];
	long yen = 100000;
	long long int i;
	for(i=0;i<n;i++){
		visit[i] = 0;
	}
	for(i=0;i<n;i++){
		scanf("%lld %lld",&store[i][0],&store[i][1]);
	}
	long int buff_store;
	while(buy < m) {
		for(i=0;i<n;i++){
			if(visit[i] == 0){
				if(store[i][0] < yen) {
					yen = store[i][0]; 
					buff_store = i;
				}
			} 
		}
		if(m-buy < store[buff_store][1]) {
			sum +=(m-buy) * store[buff_store][0];
		}
		else sum += store[buff_store][0]*store[buff_store][1];
		buy += store[buff_store][1];
		visit[buff_store] = 1;
		yen = 100000;
	}
	printf("%lld\n",sum);
	
	return 0;
}