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 9.2.1) (judge verdict: Runtime Error)

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

typedef long long ll;

int max(int a, int b){
  if(a > b) return a;
  else return b;
}
int min(int a, int b){
  if(a < b) return a;
  else return b;
}

  typedef struct{
    int value;
    int num;
  }str;

int sort(const void *a ,const void *b){
  return ((str*)a)->value - ((str*)b)->value;
}

int main(){

  int n,m;
  scanf("%d%d",&n,&m);

  str Array[100];

  for(int i = 0; i < n; ++i){
    scanf("%d %d",&Array[i].value, &Array[i].num);
  }

  qsort(Array ,n, sizeof(str), sort);

  ll sum = 0,count = 0;
  while(m > 0){
    if(m > Array[count].num){
      sum +=  Array[count].num * Array[count].value;
      m -= Array[count].num;
    }else{
      sum += m *Array[count].value;
      m -= Array[count].num;
    }
    count ++;
  }

  printf("%lld",sum);

  return 0;
}