#include <bits/stdc++.h>
using namespace std;
int main() {
  int t = 0;
  int l, r, n, m, x;
  cin >> n >> x;
  int sum;
  int temp = 0;
  for (int i = 1; i <= n; i++) {
    cin >> l >> r;
    sum = 0;
    while (1) {
      t = t + x;
      if (t >= l) {
        t = (t - x);
        sum = sum + (r - t);
        t = t + sum;
        break;
      }
    }
    temp = temp + sum;
  }
  cout << temp << endl;
}
