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 <iostream>
#include <iomanip>
#include <string>
#include <map>
#include <math.h>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <set>
#include <queue>
#include <sstream>
#include <stack>
#include <deque>
#include <memory.h>
#include <cassert>
#include <ctime>
#include <climits>
#include <list>
#include <time.h>
#include <unordered_map>
#include <bitset>
#include <exception>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef unsigned long long ull;
#define pb push_back
#define mp make_pair
#define FILE "file"
#define lc v << 1
#define rc (v << 1) + 1
#define inf 1e+9
#define linf ll(1e+18)
#define dinf 1e+14

typedef long long ll;
#define pb push_back
#define mp make_pair


vector<pair<ll, ll>> v;
bool cmp(pair<ll, ll> a, pair<ll, ll> b)
{
    return a.first < b.first;
}

void solve()
{
    int n, m;
    cin >> n >> m;
    
    v.resize(n);
    for(int i = 0; i < n; i++)
    {
        cin >> v[i].first >> v[i].second;
    }
    
    sort(begin(v), end(v), cmp);
    
    ll ans = 0;
    
    ll cnt = 0;
    int i = 0;
    while(cnt < m)
    {
        ll temp = min(m - cnt, v[i].second);
        cnt += temp;
        ans += temp * v[i].first;
        i++;
    }
    cout << ans;
}

int main()
{
    ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    //freopen("/Users/bekhzodsoliev/Desktop/C++/cp/cp/file.in", "r", stdin);freopen("/Users/bekhzodsoliev/Desktop/C++/cp/cp/file.out", "w", stdout);
    
    int t = 1;
    //cin >> t;
    
    while(t--)
    {
        solve();
    }
    
    return 0;
}