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 Python (3.4.3) (judge verdict: Accepted)

# from math import factorial,sqrt,ceil,gcd
# from itertools import permutations as permus
# from collections import deque,Counter
# import re
# from functools import lru_cache # 簡単メモ化 @lru_cache(maxsize=1000)
# from decimal import Decimal, getcontext
# # getcontext().prec = 1000
# # eps = Decimal(10) ** (-100)

# import numpy as np
# import networkx as nx
# from scipy.sparse.csgraph import shortest_path, dijkstra, floyd_warshall, bellman_ford, johnson
# from scipy.sparse import csr_matrix
# from scipy.special import comb

# slist = "abcdefghijklmnopqrstuvwxyz"
N,M = map(int,input().split())
arrAB = []
for i in range(N):
    arrAB.append(list(map(int,input().split())))

arrAB = sorted(arrAB, key=lambda lis: lis[0])

drink = M
ans = 0
for lis in arrAB:
    if drink<=0:
        break
    num = min(drink,lis[1])
    ans += lis[0] * num
    drink -= num

print(ans)
# print(*ans)   # unpackして出力。間にスペースが入る
# for row in board:
#     print(*row,sep="")    #unpackして間にスペース入れずに出力する
# print("{:.10f}".format(ans))
# print("{:0=10d}".format(ans))