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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#include <cstdlib>  // abs() for integer 絶対値求めやつ
#include <cmath>    // abs() for float, and fabs()
#include <algorithm>
#include <vector>
#include <string>
#include<iomanip>//浮動小数点数を出力する制度の指定
//使い方　数値をnumとして　cout<<setprecision(n)<<num ;nは桁数
#define rep(i,n) for(int i = 0; i < (n);i++)
#define SORT(a) sort((a).begin(),(a).end())
#define REV(a) reverse((a).begin(),(a).end())
using namespace std;
using ll =long long;
#define INF 2000000000
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
#define che(a,string) cout<<"//"<<string <<"=="<<(a)<<"//" <<endl;
bool IsInt(double a){int b = a/1;if(a==b){return true;}else {return false;}}
/*覚えてないことメaモ
数値型から文字列に変換　to_string(number);これは簡単
文字列から数値型に変換　stoi(number)
文字列からllに変換　stoll(number)*/

//ここから書き始める

int main(){
    int n ; cin >>n;
    int m ; cin>> m;
    vector<pair<int,int>> P(n);
    rep(i,n){
        int a,b;
        cin >> a >> b; //a円でb本まで買える
        P[i]=make_pair(a,b);
    }
    SORT(P);
    int now = 0;//買った本数
    ll ans = 0;
    int check = 0;
    for(int i = 0 ; i < n ; i++){
        int a, b;
        tie (a,b) = P[i];
        //che(a,'a');che(b,'b');
        for(int j = 0 ; j < b ; j++){
            if(now==m){
                break;
                check = 1; 
            }
            ans +=a;
            now++;
        }
        if(check){
            break;
        }
    }
    cout<<ans<<endl;
    }