Problem:
Score : 200 points

Problem Statement

We have A cards, each of which has an integer 1 written on it. Similarly, we also have B cards with 0s and C cards with -1s.

We will pick up K among these cards. What is the maximum possible sum of the numbers written on the cards chosen?

Constraints

All values in input are integers.

0 \leq A, B, C

1 \leq K \leq A + B + C \leq 2 \times 10^9

Input

Input is given from Standard Input in the following format:

A B C K

Output

Print the maximum possible sum of the numbers written on the cards chosen.

Sample Input 1

2 1 1 3

Sample Output 1

2

Consider picking up two cards with 1s and one card with a 0.
In this case, the sum of the numbers written on the cards is 2, which is the maximum possible value.

Sample Input 2

1 2 3 4

Sample Output 2

0

Sample Input 3

2000000000 0 0 2000000000

Sample Output 3

2000000000

Solution in Rust (1.42.0) (judge verdict: Wrong Answer)

// -*- coding:utf-8-unix -*-

use proconio::input;

fn main() {
    input! {
        a: usize,
        b: usize,
        c: usize,
        k: usize
    }
    // println!("{}", k);

    // aから順番に使っていくだけ
    let mut res = 0;
    let mut unuse_k = k;

    // k <= a の場合
    if  k <= a {
        res = k;
        println!("{}", res);
        return;
    }

    // k <= a + b の場合

    // aの点数を足す
    res += a;
    unuse_k -= a;

    // 残り使える枚数がbの方が多い時
    if unuse_k <= b {
        println!("{}", res);
        return;
    }

    // k <= a + b + c の場合

    unuse_k -= b;

    if unuse_k <= c {
        res -= unuse_k;
        println!("{}", res);
        return;
    }

    res -= c;
    println!("{}", res);

    // kの場合分けしたほうがわかりやすかった気がする
}