#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <chrono>
#define _USE_MATH_DEFINES
#include <cmath>
#include <cstdio>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iostream>
#include <iterator>
#include <map>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
using namespace std;

#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()

const int INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-8;
const int MOD = 1000000007; // 998244353;
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
/*-------------------------------------------------*/
int main() {
  cin.tie(0); ios::sync_with_stdio(false);
  // freopen("input.txt", "r", stdin);

  int n; long long m; cin >> n >> m;
  vector<int> a(n), b(n); REP(i, n) cin >> a[i] >> b[i];
  long long ans = 0;
  long long dp1 = 0, dp2 = 0, dp3 = 0;
  REP(i, n) {
    long long tmp1 = max({dp1, dp2, dp3}) + b[i];
    long long tmp2 = max({dp1, dp2, dp3}) + a[i];
    long long tmp3 = max(dp2, dp3) + (m - 1) * a[i];
    long long tmp4 = dp3 + (m - 1) * a[i] + b[i];
    long long tmp5 = dp3 + m * a[i];
    ans = max({ans, tmp1, tmp2, tmp3, tmp4, tmp5});
    dp1 = max({tmp1, tmp2, tmp3, tmp4, tmp5});
    dp2 = max({tmp1, tmp3, tmp4, tmp5});
    dp3 = max({tmp1, tmp4, tmp5});
  }
  cout << ans << '\n';
  return 0;
}