#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

ll N, D;
ll T[110], K[110];
ll dp[110][2];

int main() {
  cin >> N >> D;
  for (int i = 0; i < N; i++) {
    cin >> T[i] >> K[i];
  }

  dp[0][1] = -(1LL << 60);
  for (int i = 0; i < N; i++) {
    dp[i + 1][0] = max(dp[i][0] + T[i], dp[i][1] + T[i] - D);
    dp[i + 1][1] = max(dp[i][0] + K[i] - D, dp[i][1] + K[i]);
  }
  cout << max(dp[N][0], dp[N][1]) << endl;

  return 0;
}