package main

import (
    "fmt"
    "math"
)

func main() {
    var n, ans int
    fmt.Scan(&n)
    max := -1
    for i := 0; i < n; i++ {
        var a, b int
        fmt.Scan(&a, &b)
        w := a + 4 * b

        if int(math.Abs(float64(max - w))) % 2 != 0 && max != -1{
            ans = -1
            break
        }

        if w > max {
            ans += (w - max) / 2 * i
            max = w
        } else {
            ans += (max - w) / 2
        }
    }
    fmt.Println(ans)
}