#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, m, k, i, x, y, s, p;
  cin >> n >> m;
  s = 0;
  k = 1;
  for (i = 1; i <= n; i++) {
    cin >> x >> y;
    k = x - k;
    p = k % m;
    x -= p;
    s += y - x + 1;
    k = y + 1;
  }
  cout << s << endl;
}
