#include<bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

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 T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}


int main() {
  int N, M;
  cin >> N >> M;
  vector< int64 > A(N), B(N);
  for(int i = 0; i < N; i++) {
    cin >> A[i] >> B[i];
  }
  auto get_cost = [&](int i, int t) {
    if(t == 0) return 0LL;
    return max({A[i] * t, B[i], A[i] * (t - 1) + B[i]});
  };
  // たぶん1回なにかしてM-1回何かをするべきだとおもう
  vector< int64 > S(N);
  for(int i = 0; i < N; i++) {
    S[i] = get_cost(i, M);
    if(i) S[i] += S[i - 1];
  }
  int64 sum = 0, ret = S.back();
  for(int i = N - 1; i >= 0; i--) {
    chmax(sum, 0);
    sum += get_cost(i, 1);
    if(i) chmax(ret, sum + S[i - 1]);
  }
  cout << ret << endl;
}