#include "bits/stdc++.h"
#define in std::cin
#define out std::cout
#define rep(i,N) for(LL i=0;i<N;++i)
typedef long long int LL;

LL N, M;
std::vector<LL>A, B;
std::vector<std::vector<LL>>memo;

LL dp(LL i, LL j)
{
	if (i == -1) return 0;
	if (j == 0)
	{
		LL res1 = dp(i - 1, 0);
		LL res2 = dp(i - 1, 1) + std::max(A[i], B[i]);
		LL res3 = dp(i - 1, 2) + A[i] * (M - 1) + std::max(A[i], B[i]);
		return memo[i][j] = std::max({ res1,res2,res3 });
	}
	if (j == 1)
	{
		LL res1 = dp(i - 1, 1) + std::max(A[i], B[i]);
		LL res2 = dp(i - 1, 2) + A[i] * (M - 1) + std::max(A[i], B[i]);
		return memo[i][j] = std::max({ res1,res2 });
	}
	if (j == 2)
	{
		return memo[i][j] = dp(i - 1, 2) + A[i] * (M - 1) + std::max(A[i], B[i]);
	}
}

int main()
{
	in >> N >> M;
	A.resize(N); B.resize(N);
	rep(i, N) in >> A[i] >> B[i];

	memo.resize(N + 1, std::vector<LL>(5, -1));
	out << dp(N - 1, 0) << std::endl;
}