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, cmp};

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let n: i32 = input.split_whitespace().next().unwrap().parse().unwrap();
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let k: i32 = input.split_whitespace().next().unwrap().parse().unwrap();
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let x: i32 = input.split_whitespace().next().unwrap().parse().unwrap();
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let y: i32 = input.split_whitespace().next().unwrap().parse().unwrap();

    // println!("n = {}, k = {}, x = {}, y = {}", n, k, x, y);

    let first = x * cmp::min(n, k);
    let rest = y * cmp::max(0, n - k);
    println!("{}", first + rest);
}