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>
#include <string.h>
#include <time.h>
#include <math.h>
#define FOR(i,a,n) for(i=(a);i<(n);i++)
#define swap(type,a,b) do{type t=a; a=b; b=t}while(0)
#define MAX(a,b) (((a)>(b))?(a):(b))
#define MIN(a,b) (((a)<(b))?(a):(b))
#define SIZE 100005
typedef long long ll;
const ll INF=100000000;
const ll MOD=1000000007;
typedef struct{
    int v1,v2;
}Pair;
int comp(const void *a,const void *b){
    return ((Pair*)a)->v1 - ((Pair*)b)->v1;
}
Pair p[SIZE];
int main(void){
    int n,m,i;
    ll res;
    scanf("%d%d",&n,&m);
    FOR(i,0,n) scanf("%d%d",&p[i].v1,&p[i].v2);
    qsort(p,n,sizeof(Pair),comp);
    FOR(i,0,n){
        int k=MIN(m,p[i].v2);
        res += (ll)k*(ll)p[i].v1;
        m -= k;
        if(m==0) break;
    }
    printf("%lld\n",res);
    return 0;
}