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 PyPy3 (2.4.0) (judge verdict: Wrong Answer)

n = int(input())
s = int(input())
x = int(n**0.5)+1
def dsum(x,t):
  ret = 0
  while x>0:
    ret += x%t
    x //= t
  return ret
if n == s:
  print(n+1)
  exit()
for i in range(2,x+1):
  if dsum(n,i) == s:
    print(i)
    exit()
for i in range(x,0,-1):
  if (n-(s-i))%i == 0:
    digit = (n-(s-i))//i
    if 0 < i < digit and 0 <= s-i < digit:
      print((n-(s-i))//i,i)
      exit()
print(-1)