#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
const long long LINF = 1e18;
//const bool debug = false;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, d; cin >> n >> d;
	vector<long long>p(n), q(n);
	rep(i, n)cin >> p[i] >> q[i];
	long long ans = -LINF;
	rep(i, n)rep(j, n) {
		if (i == j)continue;
		long long anssub = LINF;
		long long x = (q[i] - p[i]) + (q[j] - p[j]);
		if (x >= 0) {
			chmin(anssub, -p[i]);
			chmin(anssub, -p[i] + q[i] - p[j]);
		}
		else {
			{
				long long sub = -p[i];
				sub += x * ((d - 1) / 2);
				chmin(anssub, sub);
			}
			{
				long long sub = -p[i] + q[i] - p[j];
				sub += x * ((d - 2) / 2);
				chmin(anssub, sub);
			}
		}
		chmax(ans, anssub);
	}
	cout << ans << endl;
	return 0;
}