import Foundation

let input = readLine()!.split(separator: " ").map { Int($0)! }
let A = input[0]
let B = input[1]
let C = input[2]

let aPerJob = A * 60
let bPerJob = B
let cCreation = C * 3600

let diff = aPerJob - bPerJob

if diff <= 0 {
    print("-1")
} else {
    let ans = (cCreation + diff - 1) / diff
    print(ans)
}