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: Accepted)

#include <stdio.h>
#include <stdlib.h>
long long cmp(const void *a,const void *b){                                                                               return *(long long*)a - *(long long*)b;
}
int main(void){
  long long n,m,i,j,ans=0,a[100005][2];;                                                                                  scanf("%lld%lld",&n,&m);
  for(i=0;i<n;i++) scanf("%lld %lld",&a[i][0],&a[i][1]);
  qsort(a,n,sizeof(a[0]),cmp);                                                                                            i = 0;
  while(1){
    if(m>a[i][1]){
      m-=a[i][1];
      ans += a[i][0]*a[i][1];
    }
    else{
      ans+=m*a[i][0];
      break;
    }
    i++;
  }
  printf("%lld",ans);
  return 0;
}