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)

#[allow(dead_code)]
fn scan<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

#[allow(dead_code)]
fn scan_split<T: std::str::FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split_whitespace()
        .map(|t| t.parse().ok().unwrap()).collect()
}

#[allow(dead_code)]
fn scan_split_with<T: std::str::FromStr>(ch: char) -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().split(ch)
        .map(|t| t.parse().ok().unwrap()).collect()
}

#[allow(dead_code)]
fn scan_lines<T: std::str::FromStr>(n: usize) -> Vec<T> {
    (0..n).map(|_| scan::<T>()).collect()
}

#[allow(unused_macros)]
macro_rules! scan_as_tuple {
    ( $t:ty ) => {
        (scan::<$t>(),)
    };
    ( $( $t:ty ),* ) => {
        {
            let mut s = String::new();
            std::io::stdin().read_line(&mut s).ok();
            let v = s.trim().split_whitespace();
            let mut it = v;
            let ret = ( $(it.next().unwrap().parse::<$t>().ok().unwrap()),* );
            assert!(it.next() == None, "not all tokens consumed");
            ret
        }
    };
}

fn main() {
    let n = scan::<i32>();
    let k = scan::<i32>();
    let x = scan::<i32>();
    let y = scan::<i32>();
    
    if n <= k {
        println!("{}", x*n);
    } else {
        println!("{}", x*k+y*(n-k));
    }
}