#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
// const int mod = 1e9 + 7;
const int mod = 998244353;

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)};
}

#pragma GCC optimize ("O3")
#pragma GCC target ("avx2") // or sse4
#pragma GCC optimize("unroll-loops")

int main() {
  int N, C;
  cin >> N >> C;
  vector< int64 > A(N), B(N);
  for(int i = 0; i < N; i++) {
    cin >> A[i] >> B[i];
  }
  vector< int64 > dp(N + 1, infll);
  dp[0] = 0;
  vector< int64 > dp2(N + 1, infll);
  dp2[0] = 0;

  auto set_j = [&](int64 j, int64 v) {
    v *= 2;
    v += j * j * C;
    v += j * C;
    chmin(dp2[j], v);
  };
  for(int64 i = 1; i <= N; i++) {
    int64 ret = infll;
    for(int64 j = 0; j < i; j++) {
      chmin(ret, dp2[j] + -(2 * j * (A[i - 1] + i * C)));
    }
    ret += i * i * C;
    ret -= i * C;
    ret += i * A[i - 1] * 2;
    ret /= 2;
    ret += B[i - 1];
    for(int j = i; j <= N; j++) {
      if(chmin(dp[j], ret + (j - i) * A[i - 1] + (j - i) * (j - i + 1) / 2 * C)) {
        set_j(j, dp[j]);
      } else {
        break;
      }
    }
  }
  cout << dp[N] << "\n";
}