using System.Collections;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

internal class Program
{
    private const long mod = 998244353;
    private const long mod17 = 1000000007;
    
    public static void Main(string[] args)
    {
        var line = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
        int n = line[0], k = line[1];
        List<(int,int)> p = new List<(int,int)>(n);
        for (int i = 0; i < n; i++)
        {
            line = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
            int l = line[0], r = line[1];
            p.Add((l, r));
        }


        p.Sort();
        //Console.WriteLine("hello");
        for (int i = 0; i < n; i++)
        {
            //Console.WriteLine("{0},{1}",p[i].Item1,p[i].Item2);
        }
        var mir = mkar(n + 1, (int)mod17);
        var chkr = mkar(n + 1, n);
        for (int i = n - 1; i >= 0; i--)
        {
            mir[i] = mir[i + 1];
            chkr[i] = chkr[i + 1];
            if (mir[i] > p[i].Item2)
            {
                mir[i] = p[i].Item2;
                chkr[i] = i;
            }
        }

        List<List<int>> nex = new List<List<int>>(n+1);
        for (int i = 0; i < n+1; i++)
        {
            var ad = mkar(18, n);
            nex.Add(ad);
        }
        
        for (int i = 0; i < n; i++)
        {
            var t = (p[i].Item2, -1);
            var loc = ~p.BinarySearch(t, new lower_bound<(int, int)>());
            //Console.WriteLine(loc);
            nex[i][0] = chkr[loc];
        }
        //Console.WriteLine("hello");
        nex[n][0] = n;
        for (int j = 0; j < 17; j++)
        {
            for (int i = 0; i < n+1; i++)
            {
                //Console.WriteLine(nex[i].Count());
                nex[i][j + 1] = nex[nex[i][j]][j];
            }
            //Console.WriteLine("Hello");
        }
        
        int ans=(int)mod17;
        for(int i=0;i<n;i++)
        {
            int cur = i;
            for (int j = 0; j < 18; j++)
            {
                if (((k - 1) & (1 << j)) != 0) cur = nex[cur][j];
            }

            if (cur < n)
            {
                int val = p[cur].Item2 - p[i].Item1;
                ans = Math.Min(ans, val);
            }
        }

        if (ans == mod17) ans = -1;
        Console.WriteLine(ans);
    }
    public class lower_bound<T> : IComparer<T> where T : IComparable<T>
    {
        public int Compare(T x, T y)
        {
            return 0 <= x.CompareTo(y) ? 1 : -1;
        }
    }
    public static List<int> mkar(int n, int val)
    {
        List<int> res = new List<int>(n);
        for(int i=0;i<n;i++)res.Add(val);
        return res;
    }
   
}