#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, x, ans = 0, a, b, t = 1;
  cin >> n >> x;
  while (n--) {
    cin >> a >> b;
    while (t <= a) t = t + x;
    t = t - x;
    ans = ans + b - t + 1;
    t = b + 1;
  }
  cout << ans;
  return 0;
}
