#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);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}

int main() {
  int N, M;
  cin >> N >> M;
  vector< int > A(N + 1), B(N + 1);
  for(int i = 0; i < N; i++) {
    int a, b;
    cin >> a >> b;
    A[i] = a, B[i] = b;
  }
  int ans = 0;

  vector< int > xs;
  for(int i = 0; i < N; i++) {
    for(int j = -1; j <= 1; j++) {
      xs.emplace_back(A[i] + j);
      xs.emplace_back(B[i] + j);
    }
  }
  sort(begin(xs), end(xs));
  xs.erase(unique(begin(xs), end(xs)), end(xs));

  {
    vector< int > add(xs.size());
    for(int i = 0; i < N; i++) {
      A[i] = lower_bound(begin(xs), end(xs), A[i]) - begin(xs);
      B[i] = lower_bound(begin(xs), end(xs), B[i] + 1) - begin(xs);
      add[A[i]]++;
      add[B[i]]--;
    }
    for(int i = 1; i < xs.size(); i++) {
      add[i] += add[i - 1];
    }
    ans = *max_element(begin(add), end(add));
  }


  auto solve = [&]() {
    int ok, ng;
    if(ans % 2 == 0) {
      ok = 0;
      ng = xs.size();
    } else {
      ok = xs.size();
      ng = 0;
    }
    auto check = [&](int md) {
      if(ans % 2 == 0) {
        A[N] = 0;
        B[N] = md - 1;
      } else {
        A[N] = md + 1;
        B[N] = xs.size();
      }

      vector< int > add(xs.size() + 1);
      for(int i = 0; i <= N; i++) {
        add[A[i]]++;
        add[B[i]]--;
      }
      for(int i = 1; i < xs.size(); i++) {
        add[i] += add[i - 1];
      }
      return *max_element(begin(add), end(add)) == ans;
    };
    while(abs(ng - ok) > 1) {
      int md = (ok + ng) / 2;
      if(check(md)) ok = md;
      else ng = md;
    }
    return ok;
  };

  if(ans % 2 == 0) {
    auto val = solve();
    cout << 1LL * (ans - 1) * M + (M - xs[val - 1]) << endl;
  } else {
    auto val = solve();
    cout << 1LL * (ans - 1) * M + xs[val] << endl;
  }
}









Measured memory_bytes: 13804