#include <cstdio>
#include <vector>
#include <string>

using namespace std;

int main() {
  int N, D;
  scanf("%d%d", &N, &D);
  vector<int> T, K;
  for (int i = 0; i < N; ++i) {
    int ti, ki;
    scanf("%d%d", &ti, &ki);
    T.push_back(ti);
    K.push_back(ki);
  }
  string current_location = "Tokyo";
  int real_tokyo_payment;
  int real_kyoto_payment;
  int salary = 0;
  for (int i = 0; i < N; ++i) {
    real_tokyo_payment = current_location == "Tokyo" ? T[i] : T[i] - D;
    real_kyoto_payment = current_location == "Kyoto" ? K[i] : K[i] - D;
    if (real_tokyo_payment >= real_kyoto_payment) {
      salary += real_tokyo_payment;
      current_location = "Tokyo";
    } else {
      salary += real_kyoto_payment;
      current_location = "Kyoto";
    }
  }
  printf("%d\n", salary);
}