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)

#include<stdio.h>

int main(){
    long int N, M;
    long int A[120000], B[120000];
    int i, j;
    long int ans, tmp;

    scanf("%d %d", &N, &M);

    for(i=0; i<N; i++){
        scanf("%d %d", &A[i], &B[i]);
    }

    for(i=0; i<N; i++){
        for(j=i+1; j<N; j++){
            if(A[i] > A[j]){
                tmp = A[i];
                A[i] = A[j];
                A[j] = tmp;
                tmp = B[i];
                B[i] = B[j];
                B[j] = tmp;
            }
        }
    }
    tmp = M;

    for(i=0; i<N; i++){
        tmp = tmp - B[i];
        if(tmp >= 0){
            ans += A[i]*B[i];
        }else{
            ans += A[i]*(tmp+B[i]);
            break;
        }
    }
    printf("%ld", ans);

    return 0;
}