import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static string[] s_rd;
T RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
string RDR()() { return readln.chomp; }
T[] ARR(T = long)(in string str, T fix = 0) { auto r = str.split.to!(T[]); r[] += fix; return r; }
size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
bool minimize(T)(ref T x, T y) { if (x > y) { x = y; return true; } else { return false; } }
bool maximize(T)(ref T x, T y) { if (x < y) { x = y; return true; } else { return false; } }

long mod = 10^^9 + 7;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }

void main()
{	
	auto N = RD;
	auto M = RD;
	auto ab = new long[2][](N);
	auto bests = new long[](N+1);
	foreach (i; 0..N)
	{
		auto A = RD;
		auto B = RD;
		ab[i] = [A, B];
		if (A > 0)
		{
			if (B > A)
			{
				bests[i+1] = A * (M-1) + B;
			}
			else
			{
				bests[i+1] = A * M;
			}
		}
		else if (A * M > B)
		{
			bests[i+1] = A * M;
		}
		else
		{
			bests[i+1] = B;
		}
		bests[i+1] += bests[i];
	}

	auto bests2 = new long[](N+1);
	foreach (i; 0..N)
	{
		bests2[i+1] = max(ab[i][0], ab[i][1]);
		bests2[i+1] += bests2[i];

	}
	auto bests22 = new long[](N+1);
	bests22[N] = bests2[N];
	foreach_reverse (i; 0..N)
	{
		bests22[i] = max(bests22[i+1], bests2[i]);
	}

	long ans = long.min;
	foreach (i; 0..N)
	{
		long x;
		if (M == 1)
			x = bests[i+1];
		else
			x = bests[i+1] + bests22[i+1] - bests2[i+1];
		ans = max(ans, x);
	}

	debug writeln(bests);
	debug writeln(bests2);

	writeln(ans);
	stdout.flush();
	debug readln();
}