if __name__=='__main__':

    st, ed = map(int,input().split())

    n = int(input())
    ra = []
    for i in range(n):
        a,b = map(int,input().split())
        ra.append([a,b])

    ra.sort()

    res = 0
    success = False
    i = 0
    while  i < n:
        j = i
        r = -float('inf')
        while j < n and ra[j][0] <= st:
            r = max(r,ra[j][1])
            j += 1

        if r < st:
            break

        res += 1
        if r >= ed:
            success = True
            break

        st = r
        i = j - 1
        i += 1

    print(res if success else -1)