// includes
#include <bits/stdc++.h>

// macros
#define ll long long int
#define pb emplace_back
#define mk make_pair
#define pq priority_queue
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define rep(i, n) FOR(i, 0, n)
#define rrep(i, n) for(int i=((int)(n)-1);i>=0;i--)
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end())
#define FI first
#define SE second
using namespace std;

//  types
typedef pair<int, int> P;
typedef pair<ll, int> Pl;
typedef pair<ll, ll> Pll;
typedef pair<double, double> Pd;
 
// constants
const int inf = 1e9;
const ll linf = 1LL << 50;
const double EPS = 1e-10;
const int mod = 1e9 + 7;

// solve
template <class T>bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;}
template <class T>bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;}
template <typename T> istream &operator>>(istream &is, vector<T> &vec){for(auto &v: vec)is >> v; return is;}

ll dp[100001][3];

int main(int argc, char const* argv[])
{
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, m;
  cin >> n >> m;
  vector<ll> a(n), b(n);
  rep(i, n)cin >> a[i] >> b[i];
  dp[1][0] = - linf;
  dp[1][1] = - linf;
  dp[1][2] = max(b[0], (m-1) * a[0] + max(a[0], b[0]));
  for(int i = 1; i < n; i++){
    dp[i+1][0] = - linf;
    rep(j, 3)dp[i+1][0] = max(dp[i+1][0], dp[i][j]);
    dp[i+1][1] = - linf;
    FOR(j, 1, 3)dp[i+1][1] = max(dp[i+1][1], dp[i][j]);
    dp[i+1][1] += max(a[i], b[i]);
    dp[i+1][2] = dp[i][2];
    dp[i+1][2] += max(b[i], (m-1) * a[i] + max(a[i], b[i]));
  }
  cout << max(dp[n][0], max(dp[n][1], dp[n][2])) << endl;
	return 0;
}