#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <string>
#include <sstream>
#include <complex>
#include <vector>
#include <list>
#include <queue>
#include <deque>
#include <stack>
#include <map>
#include <set>

#define rep(i, n) for (int i = 0; i < (n); i++)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; i--)
#define reps(i, n) for (int i = 1; i <= (n); i++)
#define repi(i, a, b) for (int i = (a); i < (b); i++)
#define all(a) (a).begin(), (a).end()
#define bit(b) (1ull << (b))

using namespace std;
using i32 = long long;
using u32 = unsigned long long;
using i64 = long long;
using u64 = unsigned long long;
using f64 = double;
using vi32 = vector<i32>;
using vu32 = vector<u32>;
using vi64 = vector<i64>;
using vu64 = vector<u64>;
using vf64 = vector<f64>;
using vstr = vector<string>;
using vvi32 = vector<vi32>;
using vvu32 = vector<vu32>;
using vvi64 = vector<vi64>;
using vvu64 = vector<vu64>;
using vvf64 = vector<vf64>;

template<typename T, typename S> void amax(T &x, S y) { if (x < y) x = y; }
template<typename T, typename S> void amin(T &x, S y) { if (y < x) x = y; }

i64 dp[110][2]; // i日目, j=0:東京,1:京都

class City {
public:
  int t, k;
};

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(16);
  int n, d;
  cin >> n >> d;
  dp[0][0] = 0;
  dp[0][1] = -d;
  vector<City> cities;
  rep(i, n) {
    int t, k;
    cin >> t >> k;
    cities.push_back({t, k});
  }
  rep(i, n) {
    dp[i + 1][0] = max(dp[i][0] + cities[i].t, dp[i][1] + cities[i].k - d);
    dp[i + 1][1] = max(dp[i][1] + cities[i].k, dp[i][0] + cities[i].t - d);
  }
  cout << max(dp[n][0], dp[n][1]) << endl;
  return 0;
}