fun main() {
    val builder = StringBuilder()

    val (a, b, c) = readInputLine().split(" ").map { it.toLong() }
    val divisor = 60 * a - b

    if (divisor <= 0L) {
        builder.appendLine(-1)
    } else {
        builder.appendLine((3600 * c + (divisor - 1L)) / divisor)
    }

    print(builder.toString())
}

fun readInputLine(): String {
    return readLine()!!
}