//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<n; ++i)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
using namespace std;
using ll = int64_t;
using ld = long double;
using P = pair<int, int>;
using vs = vector<string>;
using vi = vector<int>;
using vvi = vector<vi>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQG = priority_queue<T, vector<T>, greater<T> >;
const int INF = 100010001;
const ll LINF = (ll)INF*INF*10;
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true);}
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true);}
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second;}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << ' ' << p.second;}

const int X = 1e5+10;

//head

int n, m;
ll a[X], b[X];
ll n1, nm;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> m;
  rep(i, n) cin >> a[i] >> b[i];

  for(int i = n-1; i >= 0; i--) {
    n1 += max(a[i], b[i]);
    nm += max(max(a[i], b[i])+a[i]*(m-1), b[i]);
    if(i) {
      chmax(n1, 0);
      chmax(nm, n1);
    }
  }

  cout << nm << endl;
}