package Yukicoder

/**
 * Created by hichikawa on 2015/11/12.
 */
fun main(args: Array<String>) {
    fun readLineLongArray() : List<Long> {
        val str = readLine() as String
        val arrStr = str.split(" ")
        val ret = arrStr.map { it.toLong() }
        return ret
    }
    fun readLineLong() : Long {
        val str = readLine() as String
        return str.toLong()
    }
    fun readLineInt() : Int {
        val str = readLine() as String
        return str.toInt()
    }
    fun readLineIntArray() : List<Int> {
        val str = readLine() as String
        val arrStr = str.split(" ")
        val ret = arrStr.map { it.toInt() }
        return ret
    }

    val num = readLineInt()
    var countSpace = 0.toLong() as Long
    var maxNum = Integer.MIN_VALUE
    for (i in 0..(num-1)) {
        val inputArrInt = readLineIntArray()
        val lenLine = inputArrInt[0] + inputArrInt[1] * 4
        if (maxNum == Integer.MIN_VALUE) {
            maxNum = lenLine
        } else if (maxNum == lenLine) {
        } else if (maxNum > lenLine) {
            if (maxNum % 2 == lenLine % 2) {
                countSpace += ((maxNum - lenLine) / 2).toLong()
            } else {
                println(-1)
                return
            }
        } else {
            if (maxNum % 2 == lenLine % 2) {
                countSpace += ((lenLine - maxNum) * i / 2).toLong()
                maxNum = lenLine
            } else {
                println(-1)
                return
            }
        }
    }
    println(countSpace)
}