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)

use std::io;
use std::str::FromStr;

fn main() {
    let stdin = io::stdin();

    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();

    let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
    let n = it.next().unwrap();

    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();

    let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
    let k = it.next().unwrap();

    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();

    let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
    let x = it.next().unwrap();

    let mut buf = String::new();
    stdin.read_line(&mut buf).ok();

    let mut it = buf.split_whitespace().map(|n| i64::from_str(n).unwrap());
    let y = it.next().unwrap();

    if n <= k {
        println!("{}", n * x);
        return;
    }
    // println!("{}", k * x);
    // println!("{}", ((n - k) * y));
    let ans = k * x + (n - k) * y;
    println!("{}", ans);
}