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;

fn main()
{
    let mut n_str = String::new();
    let mut k_str = String::new();
    let mut x_str = String::new();
    let mut y_str = String::new();

    io::stdin().read_line(&mut n_str).expect("");
    io::stdin().read_line(&mut k_str).expect("");
    io::stdin().read_line(&mut x_str).expect("");
    io::stdin().read_line(&mut y_str).expect("");

    let n:u32 = n_str.trim().parse().expect("");
    let k:u32 = k_str.trim().parse().expect("");
    let x:u32 = x_str.trim().parse().expect("");
    let y:u32 = y_str.trim().parse().expect("");

    let   to_k = x * if n > k { k   } else { n };
    let from_k = y * if n > k { n-k } else { 0 };

    println!("{}", to_k + from_k);
}