using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    static long[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, m) = (c[0], c[1]);
        var map = NArr(n);
        var left = new long[n];
        for (var i = 0; i < n; ++i)
        {
            if (i > 0) left[i] = left[i - 1];
            left[i] += Math.Max(map[i][1], map[i][0] * (m - 1) + Math.Max(map[i][0], map[i][1]));
        }
        var reat = 0L;
        var ans = 0L;
        for (var i = n - 1; i >= 0; --i)
        {
            ans = Math.Max(ans, left[i] + Math.Max(reat, 0));
            reat += Math.Max(map[i][0], map[i][1]);
        }
        WriteLine(ans);
    }
}