#include<iostream>
#include<vector>
#include<string>
#include<cmath>
#include<algorithm>
#include<functional>
#include<numeric>
#include<queue>
#include<stack>
#include<map>
#include<unordered_map>
#include<set>
#include<bitset>
#include<random>

#pragma region
using namespace std;
#define FOR(i,r,n) for(ll i = (ll)(r); i < (ll)(n); i++)
#define rep(i,n) FOR(i,0LL,n)
#define RFOR(i,r,n) for(ll i=(ll)(n-1);i>=r;i--)
#define rrep(i,n) RFOR(i,0LL,n)
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define COUNT(a,y,x) upper_bound(all(a), y) - lower_bound(all(a), x)
#define UNIQUE(a) sort(all(a)); a.erase(unique(all(a)), a.end())
#define pb push_back
typedef long long int ll;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<string> vs;
typedef map<ll, ll> MAP;
const ll inf = 2222222222222222222LL;
const ll mod = 1000000007LL;

ll n = 0, m = 0, ans = 0, tmp = 0, ma = -inf, mi = inf;
string s;
bool ok;
ll dx[9] = { 0,1,0,-1,0,1,1,-1,-1 }, dy[9] = { 0,0,1,0,-1,1,-1,1,-1 };
#define endl '\n'
#pragma endregion
#define MAX 222222



ll a[MAX], b[MAX], sum[MAX], mx[MAX];



int main(void) {
  ios::sync_with_stdio(false); cin.tie(0);



  cin >> n >> m;
  rep(i, n) cin >> a[i] >> b[i];
  ans = -inf;
  rep(i, n) {
    if (i) sum[i] = sum[i - 1];
    sum[i] += max(a[i], b[i]);
  }
  mx[n] = sum[n - 1];
  RFOR(i, 0, n) mx[i] = max(mx[i + 1], sum[i]);
  ll cur = 0;
  rep(i, n) {
    if (a[i] < 0 && b[i] < 0) {
      if (b[i] < a[i]) {
        if (b[i] > a[i] * m) {
          cur += b[i];
          cur -= a[i];
        }
      }
    }
    else {
      if (a[i] > 0) cur += a[i] * (m - 1);
    }
    ans = max(ans, cur + mx[i]);
  }

  cout << ans << endl;



  return 0;
}