package main

import(
    "fmt"
    "math"
    "sort"
)

type Pair struct {
    l int
    r int
}

type PairArray []*Pair

func (p PairArray) Len() int {
    return len(p)
}

func (p PairArray) Less(i, j int) bool {
    return p[i].l < p[j].l
}

func (p PairArray) Swap(i, j int) {
    (p)[i], (p)[j] = (p)[j], (p)[i]
}

func main() {
    var start, end int
    fmt.Scanf("%d", &start)
    fmt.Scanf("%d", &end)

    var n, l, r int
    fmt.Scanf("%d", &n)

    pairarray := make([]*Pair, n)
    for i := 0; i < n; i++ {
        fmt.Scanf("%d%d", &l, &r)
        pairarray[i] = &Pair{
            l: l,
            r: r,
        }
    }
    sort.Sort(PairArray(pairarray))

    max := math.MinInt32
    succ := false
    res := 0
    for i := 0; i < n; i++ {
        j := i
        for ; j < n && pairarray[j].l <= start; j++ {
            if max < pairarray[j].r {
                max = pairarray[j].r
            }
        }

        res++
        if max < start {
            break
        }

        if max >= end {
            succ = true
            break
        }

        start = max
        i = j-1
    }

    if succ {
        fmt.Printf("%d\n", res)
        return
    }
    fmt.Printf("-1\n")
}