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 (Clang 3.8.0) (judge verdict: Wrong Answer)

#include "stdio.h"

int N, M, A[100000], B[100000];
int a = 0;

void load();
void sort();
void calc();

int main(void){
  load();
  sort();
  calc();
  printf("%d", a);
}

void load(void){
  scanf("%d %d\n", &N, &M);
  for(int i = 0;i < N;i++){
    scanf("%d %d\n", &A[i], &B[i]);
  }
}

void sort(void){
  int buf;
  for(int i = 0;i < N;i++){
    for(int j = i + 1;j < N;j++){
      if(A[i] > A[j]){
        buf = A[i];
        A[i] = A[j];
        A[j] = buf;
        buf = B[i];
        B[i] = B[j];
        B[j] = buf;
      }
    }
  }
}

void calc(void){
  int b = 0;
  for(int i = 0;i < N;i++){
    if(b + B[i] < M){
      b += B[i];
      a += A[i] * B[i];
    }else{
      a += A[i] * (M - b);
      break;
    }
  }
}