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 Rust (1.15.1) (judge verdict: Accepted)

fn main() {
    let n: usize = atcoder::read_single_num();
    let k: usize = atcoder::read_single_num();
    let x: usize = atcoder::read_single_num();
    let y: usize = atcoder::read_single_num();

    let cost = if k < n {
        k * x + (n - k) * y
    } else {
        n * x
    };

    println!("{}", cost);
}

#[allow(dead_code)]
mod atcoder {
    pub fn read_single_num<T: ::std::str::FromStr>() -> T
        where <T as ::std::str::FromStr>::Err: ::std::fmt::Debug {
            let mut buffer = String::new();
            ::std::io::stdin().read_line(&mut buffer).unwrap();
            buffer.trim().parse().unwrap()
    }

    pub fn read_multiple_num<T: ::std::str::FromStr>() -> Vec<T>
        where <T as ::std::str::FromStr>::Err: ::std::fmt::Debug {
            let mut buffer = String::new();
            ::std::io::stdin().read_line(&mut buffer).unwrap();
            buffer.split_whitespace().map(|i| i.parse().unwrap() ).collect()
    }

    pub fn read_single_string() -> String {
            let mut buffer = String::new();
            ::std::io::stdin().read_line(&mut buffer).unwrap();
            buffer.trim().to_string()
    }

    pub fn read_multiple_string() -> Vec<String> {
            let mut buffer = String::new();
            ::std::io::stdin().read_line(&mut buffer).unwrap();
            buffer.split_whitespace().map(|s| s.to_string() ).collect()
    }
}