use std::cmp::*; use std::io::Read; fn get_word() -> String { let stdin = std::io::stdin(); let mut stdin=stdin.lock(); let mut u8b: [u8; 1] = [0]; loop { let mut buf: Vec = Vec::with_capacity(16); loop { let res = stdin.read(&mut u8b); if res.unwrap_or(0) == 0 || u8b[0] <= b' ' { break; } else { buf.push(u8b[0]); } } if buf.len() >= 1 { let ret = String::from_utf8(buf).unwrap(); return ret; } } } #[allow(dead_code)] fn get() -> T { get_word().parse().ok().unwrap() } // DP table indexed by {floor(n / i) | 1 <= i <= n}. // Verified by: https://atcoder.jp/contests/abc239/submissions/29553511 type K = i64; type V = i64; struct QuoDP { // stores dp[n], dp[n/2], ..., dp[n/b]. dp_big: Vec, dp: Vec, n: K, b: K, } impl QuoDP { pub fn new(n: K, b: K) -> Self { let dp_big = vec![0.into(); b as usize + 1]; let dp = vec![0.into(); ((n + b - 1) / b) as usize]; QuoDP { dp_big: dp_big, dp: dp, n: n, b: b, } } #[allow(unused)] pub fn keys(&self) -> Vec { (1..(self.n + self.b - 1) / self.b).chain((1..=self.b).rev().map(|x| (self.n + x - 1) / x)).collect() } // pos should be of form ceil(n / ???). pub fn upd(&mut self, pos: K, f: F) where F: Fn(V) -> V { if pos >= (self.n + self.b - 1) / self.b { let idx = (self.n + pos - 1) / pos; debug_assert_eq!(pos, (self.n + idx - 1) / idx); self.dp_big[idx as usize] = f(self.dp_big[idx as usize]); return; } let idx = pos as usize; self.dp[idx] = f(self.dp[idx]); } pub fn get(&self, pos: K) -> V { if pos >= (self.n + self.b - 1) / self.b { let idx = (self.n + pos - 1) / pos; debug_assert_eq!(pos, (self.n + idx - 1) / idx); return self.dp_big[idx as usize]; } let idx = pos as usize; self.dp[idx] } #[allow(unused)] pub fn init(&mut self, f: F) where F: Fn(K) -> V { self.upd_all(|k, _| f(k)); } pub fn upd_all(&mut self, f: F) where F: Fn(K, V) -> V { for i in 0..self.dp.len() { self.dp[i] = f(i as K, self.dp[i]); } for i in (1..self.dp_big.len()).rev() { self.dp_big[i] = f((self.n + i as K - 1) / i as K, self.dp_big[i]); } } } impl std::fmt::Debug for QuoDP { fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result { for i in 0..self.dp.len() { writeln!(f, "{}: {}", i, self.dp[i])?; } for i in (1..self.dp_big.len()).rev() { writeln!(f, "{}: {}", (self.n + i as K - 1) / i as K, self.dp_big[i])?; } Ok(()) } } // Tags: variant-of-lucys-sieve // Complexity: O(n^{3/4}) fn main() { let a: i64 = get(); let b: i64 = get(); let n: i64 = get(); let inf = a + (n - 1) * b; let mut sqn = 0; while sqn * sqn <= n { sqn += 1; } sqn -= 1; let mut dp = QuoDP::new(n, sqn); let keys = dp.keys(); dp.upd(1, |_| 0); for pos in keys { if pos == 1 { continue; } let mut x = 1; while x * x <= pos { x += 1; } x -= 1; let mut tmp = inf; // Iterate over ceil(pos / i) for 1 <= i <= pos for j in 2..=x { tmp = min(tmp, dp.get((pos + j - 1) / j) + a + (j - 1) * b); } for j in 1..pos / x { let l = (pos + j - 1) / j; tmp = min(tmp, dp.get(j) + a + (l - 1) * b); } dp.upd(pos, |_| tmp); } println!("{}", dp.get(n)); }