#include <bits/stdc++.h>
using namespace std;
const long long INF = 1000000000;
int main(){
  int N, D;
  cin >> N >> D;
  vector<vector<int>> T(N, vector<int>(2));
  for (int i = 0; i < N; i++){
    cin >> T[i][0] >> T[i][1];
  }
  vector<vector<long long>> dp(N + 1, vector<long long>(2, -INF));
  dp[0][0] = 0;
  for (int i = 0; i < N; i++){
    for (int j = 0; j < 2; j++){
      dp[i + 1][j] = max(dp[i + 1][j], dp[i][j] + T[i][j]);
      dp[i + 1][j] = max(dp[i + 1][j], dp[i][1 - j] - D + T[i][j]);
    }
  }
  cout << max(dp[N][0], dp[N][1]) << endl;
}