#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;

int main()
{
	LL N, M;
	in >> N >> M;
	std::vector<LL>A(N), B(N);
	rep(i, N) in >> A[i] >> B[i];

	std::vector<LL>A_imos(N + 1), max_imos(N + 1);
	rep(i, N)
	{
		A_imos[i + 1] = A_imos[i] + A[i];
		max_imos[i + 1] = max_imos[i] + std::max(A[i], B[i]);
	}

	LL maxN = 0, maxP = 0, ans = 0;
	for (LL i = 0; i <= N; ++i)
	{
		if (maxN < A_imos[i])
		{
			maxN = A_imos[i];
			maxP = i;
		}
	}
	ans += maxN * (M - 1);
	maxN = 0, maxP = 0;
	for (LL i = 0; i <= N; ++i)
	{
		if (maxN < max_imos[i])
		{
			maxN = max_imos[i];
			maxP = i;
		}
	}
	ans += maxN;

	out << ans << std::endl;
}