#include <bits/stdc++.h>
using namespace std;
int n, x, l[60], r[60], ans;
int main() {
  scanf("%d %d", &n, &x);
  for (int i = 1; i <= n; ++i) {
    scanf("%d %d", &l[i], &r[i]);
  }
  for (int i = 1; i <= n; ++i) {
    ans += (l[i] - r[i - 1] - 1) % x;
    ans += r[i] - l[i] + 1;
  }
  printf("%d", ans);
}
