using static System.Math;
using System;

public class Hello
{
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var d = int.Parse(line[1]);
        var p = new int[n ];
        var q = new int[n ];
        for (int i = 0; i < n; i++)
        {
            line = Console.ReadLine().Trim().Split(' ');
            p[i] = int.Parse(line[0]);
            q[i] = int.Parse(line[1]);
        }
        getAns(n, d, p, q);

    }
    static void getAns (int n, int d, int [] p, int[] q)
    {
        var ok = -100000000;
        var ng = 0;
        while (ng -ok > 1)
        {
            var mid = ok + (ng - ok) / 2;
            if (check(n, d, p, q, mid)) ok = mid;
            else ng = mid;
        }
        Console.WriteLine(ok);
    }

    static bool check (int n, int d, int[]p, int[]q ,  int t)
    {
        var dp = new int[d + 1, n];
        for (int i = 0; i < d + 1; i++)
            for (int j = 0; j < n; j++) dp[i, j] = int.MinValue;
        for (int i = 0; i < n; i++)
            if (-p[i] >= t) dp[1, i] = -p[i] + q[i];
        for (int i = 2; i <= d; i++)
        {
            for (int j = 0; j < n; j++)
            {
                for (int k = 0; k < n; k++)
                {
                    if (j == k) continue;
                    if (dp[i - 1, j] == int.MinValue) continue;
                    var temp = dp[i - 1, j] - p[k];
                    if (temp < t) continue;
                    temp += q[k];
                    dp[i, k] = Max(dp[i, k], temp);
                }
            }
        }
        for (int i = 0; i < n; i++)
            if (dp[d,i] != int.MinValue) { return true; }
        return false;
    }
}