// includes #include // 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 P; typedef pair Pl; typedef pair Pll; typedef pair Pd; // constants const int inf = 1e9; const ll linf = 1LL << 50; const double EPS = 1e-10; const int mod = 1e9 + 7; // solve template bool chmax(T &a, const T &b){if(a < b){a = b; return 1;} return 0;} template bool chmin(T &a, const T &b){if(a > b){a = b; return 1;} return 0;} template istream &operator>>(istream &is, vector &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 a(n), b(n); rep(i, n)cin >> a[i] >> b[i]; for(int i = 0; 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; }