// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------

use std::io::Write;
use std::collections::*;

type Map<K, V> = BTreeMap<K, V>;
type Set<T> = BTreeSet<T>;
type Deque<T> = VecDeque<T>;

fn run() {
    input! {
        a: u64,
        b: u64,
        n: u64,
    }
    if n == 1 {
        println!("0");
        return;
    }
    let mut ans = a + (n - 1) * b;
    for i in 1..=30 {
        let valid = |m: u64| -> bool {
            let m = m;
            let q = m / (i + 1);
            let r = m % (i + 1);
            let mut a = vec![];
            for j in 0..=i {
                if j < r {
                    a.push(q + 2);
                } else {
                    a.push(q + 1);
                }
            }
            let v = a.iter().fold(1u64, |s, a| s.saturating_mul(*a)) >= n;
            v
        };
        let mut ng = i;
        let mut ok = i;
        while !valid(ok) {
            ng = ok;
            ok *= 2;
        }
        while ok - ng > 1 {
            let mid = (ok + ng) / 2;
            if valid(mid) {
                ok = mid;
            } else {
                ng = mid;
            }
        }
        ans = ans.min(a * (i + 1) + ok * b);
    }
    println!("{}", ans);
}

fn main() {
    run();
}