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<stdlib.h>

typedef struct{
    int a;
    int b;
}TEST;

int comp( const void *c1, const void *c2 )
{
  TEST test1 = *(TEST *)c1;
  TEST test2 = *(TEST *)c2;

  int tmp1 = test1.a;   /* b を基準とする */
  int tmp2 = test2.a;

  return tmp1 - tmp2;
}

int main(){
    int N, M;
    int i, j;
    double ans=0;
    TEST inp[110000];

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

    for(i=0; i<N; i++){
        scanf("%d %d", &inp[i].a, &inp[i].b);
    }

    qsort(inp, N, sizeof(TEST), comp);

    for(i=0; i<N; i++){
        if(M<inp[i].b){
            ans = ans + (double)M* inp[i].a;
            break;
        }else{
            ans = ans + (double)inp[i].b * inp[i].a;
            M = M-inp[i].b;
        }
    }

    printf("%.0lf", ans);

    return 0;
}