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 Julia (0.5.0) (judge verdict: Accepted)

MyString=Union{String,SubString{String},Char}
MyStringArray=Union{Array{String,1},Array{SubString{String},1},Array{Char,1}}
parseInt(x::MyString) = parse(Int, x)
parseInt(x::Real) = trunc(Int, x)
parseBigInt(x::MyString) = parse(BigInt, x)
parseBigInt(x::Real) = trunc(BigInt, x)
parseFloat(x::MyString) = parse(Float64, x)
parseFloat(x::Real) = trunc(Float64, x)
parseBigFloat(x::MyString) = parse(BigFloat, x)
parseBigFloat(x::Real) = trunc(BigFloat, x)
parseMap(x::MyStringArray) = map(parseInt, x)
parseMapBig(x::MyStringArray) = map(parseBigInt, x)

function main()
    n,k,x,y=[parseInt(readline()) for i in 1:4]
    if k<n
        println(k*x+(n-k)*y)
    else
        println(n*x)
    end
end
main()