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>
#include<stdlib.h>
#include<limits.h>
#include<string.h>

struct shop{
    int A;
    int B;
};

struct shop temp[10010];
void MergeSort(struct shop s[], int left, int right){
    int mid, i, j, k;
    if (left >= right) return;
    mid = (left + right) / 2;
    MergeSort(s, left, mid);
    MergeSort(s, mid + 1, right);
    for (i=left;i<=mid;i++) temp[i] = s[i];
    for (i=mid+1,j=right;i<=right;i++,j--) temp[i] = s[j];
    i=left, j=right;
    for(k=left;k<=right;k++){
        if(temp[i].A<=temp[j].A) s[k]=temp[i++];
        else s[k]=temp[j--];
    } 
}

int main(){
    long long int N,M;
    scanf("%d %d",&N,&M);
    struct shop S[N];
    long long int i,ans=0;
    for(i=0;i<N;i++) scanf("%d %d",&S[i].A,&S[i].B);
    MergeSort(S, 0, N-1);
    
    while(M>0){
        for(i=0;i<N;i++){
           // printf("%d %d\n",M, ans);
            if(M>S[i].B){
                ans += S[i].A * S[i].B;
                M -= S[i].B;
            }else if(M>0){
                ans += M * S[i].A;
                M = 0;
                i = N;
            }         
        }
    }
   // for(i=0;i<N;i++) printf("%d %d\n",S[i].A,S[i].B);
    printf("%d",ans);
    return 0;
}