import java.util.*

fun main(args: Array<String>) {
    val sc = Scanner(System.`in`)

    val hand = sc.nextInt() * 60
    val runCost = sc.nextInt()
    val initCost = sc.nextInt() * 3600

    if (hand <= runCost) {
        println(-1)
        return
    }
    println((initCost - 1) / (hand - runCost) + 1)
}