#include"bits/stdc++.h"

using namespace std;
using ll = long long;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

#define FOR(k,m,n) for(ll (k)=(m);(k)<(n);(k)++)
#define REP(i,n) FOR((i),0,(n))
#define WAITING(str) int str;std::cin>>str;
#define DEBUGING(str) cout<< #str << " " str<<endl

constexpr int INF = (1 << 30);
constexpr ll INFL = (1ll << 60);
constexpr ll MOD = 1000000007;// 10^9+7



int main()
{
	ll N, M;
	cin >> N >> M;
	vector<pair<ll, ll>> abs(N);
	REP(i, N) {
		ll a, b;
		cin >> a >> b;
		abs[i] = { a,b };
	}

	vector<ll> imos(N);
	// collection border
	REP(i, N) {
		auto ab = abs[i];
		ll a = ab.first;
		ll b = ab.second;
		if (a < 0 && b < 0) {
			if (a*M < b)imos[i] = b;
			else imos[i] = a * M;
		}
		else if (a >= 0 && b < 0) {
			imos[i] = a * M;
		}
		else if (a < 0 && b >= 0) {
			imos[i] = b;
		}
		else {
			imos[i] = a * (M - 1) + max(a, b);
		}
	}
	REP(i, N - 1)imos[i + 1] += imos[i];

	vector<ll> mid(N);
	REP(i, N)mid[i] = max(abs[i].first, abs[i].second);
	REP(i, N - 1)mid[i + 1] += mid[i];

	ll maxv = mid.back();
	vector<ll> imos2(N, 0);
	for (int i = N - 2; i >= 0; i--) {
		maxv = max(maxv, mid[i]);
		imos2[i] = maxv - mid[i];
	}

	ll res = -INFL;;
	REP(i, N)res = max(res, imos[i] + imos2[i]);
	cout << res << endl;

	return 0;
}