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: Runtime Error)

#define _USE_MATH_DEFINES 
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <unordered_map>
#include <map>
#include <math.h>
#include <unordered_set>
#include <iomanip>
#include <cmath>
#include <set>
#include <locale>
using namespace std;
typedef long long ll;
#define rep(i,n) for(int i = 0; i < (n); i++)
#define INF 2 << 29
#define MOD 1000000007
#define Yes(f) puts((f)? "Yes" : "No")
#define YES(f) puts((f)? "YES" : "NO")
int gcd(int a, int b) {
	if (b != 0)return gcd(b, a % b);
	return a;
}
ll Lgcd(ll a, ll b) {
	if (b != 0)return gcd(b, a % b);
	return a;
}
int lcm(int a, int b) {
	return a / gcd(a, b) * b;
}
ll Llcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}
using Graph = vector<vector<int>>;
//cout << fixed << setprecision(6);
//cout << setfill('0') << right << setw(2);

int main() {
	ll n, m, a, b, ans = 0, c[100100] = {};
	cin >> n >> m;
	rep(i, n) {
		cin >> a >> b;
		c[a] += b;
	}
	rep(i, 100001) {
		if (!m || !c[i]) continue;
		if (m > c[i]) {
			m -= c[i];
			ans += c[i] * i;
		}
		else {
			ans += m * i;
			m = 0;
			cout << ans << endl;
		}
	}
}