#![allow(non_snake_case)]

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut tokens = $s.split_whitespace();
        input_inner! { tokens, $($r)* }
    };

    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut res = String::new();
            ::std::io::stdin().read_to_string(&mut res).unwrap();
            res
        };
        let mut tokens = s.split_whitespace();
        input_inner! { tokens, $($r)* }
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($tokens:expr)  => {};
    ($tokens:expr,) => {};

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

#[allow(unused_macros)]
macro_rules! read_value {
    ($tokens:expr, ( $($t:tt),* )) => {
        ( $(read_value!($tokens, $t)),* )
    };

    ($tokens:expr, [ $t:tt; $len:expr ]) => {
        (0..$len).map(|_| read_value!($tokens, $t)).collect::<Vec<_>>()
    };

    ($tokens:expr, chars) => {
        read_value!($tokens, String).chars().collect::<Vec<_>>()
    };

    ($tokens:expr, usize1) => {
        read_value!($tokens, usize) - 1
    };

    ($tokens:expr, $t:ty) => {
        $tokens.next().unwrap().parse::<$t>().expect("parse error")
    };
}

fn main() {
    input! {
        A: i64,
        B: i64,
        C: i64,
    }
    // 秒単位に統一
    let A = 60*A;
    let C = 60*60*C;

    let k = A - B;
    if k <= 0 {
        println!("-1");
        return;
    }

    let ans = (C+k-1) / k;  // 切り上げ
    println!("{}", ans);
}