Problem:
Score : 100 points

Problem Statement

There is a hotel with the following accommodation fee:

X yen (the currency of Japan) per night, for the first K nights

Y yen per night, for the (K+1)-th and subsequent nights

Tak is staying at this hotel for N consecutive nights.
Find his total accommodation fee.

Constraints

1 \leq N, K \leq 10000

1 \leq Y < X \leq 10000

N,\,K,\,X,\,Y are integers.

Input

The input is given from Standard Input in the following format:

N
K
X
Y

Output

Print Tak's total accommodation fee.

Sample Input 1

5
3
10000
9000

Sample Output 1

48000

The accommodation fee is as follows:

10000 yen for the 1-st night

10000 yen for the 2-nd night

10000 yen for the 3-rd night

9000 yen for the 4-th night

9000 yen for the 5-th night

Thus, the total is 48000 yen.

Sample Input 2

2
3
10000
9000

Sample Output 2

20000

Solution in Ruby (2.3.3) (judge verdict: Wrong Answer)

def base_split(base, number, result, s)
  if number  == 0 then
    return result
  end

  result.push number % base

  if result.reduce(&:+) > s then
    return result
  end

  return base_split( base, number / base, result, s)
end

n = gets.chomp.to_i
s = gets.chomp.to_i

result = -1;
2.upto(n) {|base|
  if base_split(base, n, [], s).reduce(&:+) == s then
    result = base;
    break
  end
}

puts result