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>

#define rep(i,n,s) for(int i=s; i<n; i++)
#define ll long long

using namespace std;

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    ll n, m; cin >> n >> m;
    
    vector<pair<ll, ll>> pairs(n);
    
    for (ll i=0; i<n; i++) cin >> pairs[i].first >> pairs[i].second;
    
    sort(pairs.begin(), pairs.end());

    ll ans = 0;

    for (int i=0; i<n; i++) {
        if (pairs[i].second < m) {
            m = m - pairs[i].second;
            ans = ans + pairs[i].first * pairs[i].second;
        }
        else {
            ans += m * pairs[i].first;
            cout << ans << "\n";
            return 0;
        }
    }    


    return 0;
}