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::cmp::*;
use std::io::*;
use std::num::*;
use std::str::*;

fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let s = stdin
        .bytes()
        .map(|c| c.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect::<String>();
    s.parse::<T>().ok().unwrap()
}

fn string() -> String {
    read()
}

fn int() -> i64 {
    read()
}

fn char() -> char {
    read::<String>().pop().unwrap()
}

fn vecchar() -> Vec<char> {
    string().chars().collect()
}

fn vecint(n: i64) -> Vec<i64> {
    let mut vec = Vec::new();
    for i in 0..n {
        vec.push(int())
    }
    vec
}

fn main() {
    let (n, k, x, y) = (int(), int(), int(), int());
    if n <= k {
        println!("{}", n * x);
        return;
    }

    println!("{}", k * x + (n - k) * y)
}