#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T sq(T a) {
  return a * a;
}
template <class T, class X>
inline T _pow(T a, X y) {
  T z = 1;
  for (int i = 1; i <= y; i++) {
    z *= a;
  }
  return z;
}
template <class T>
inline T gcd(T a, T b) {
  while (b) b ^= a ^= b ^= a %= b;
  return a;
}
template <class T>
inline T lcm(T a, T b) {
  a = abs(a);
  b = abs(b);
  return (a / gcd(a, b)) * b;
}
template <class T, class X>
inline bool getbit(T a, X i) {
  T t = 1;
  return ((a & (t << i)) > 0);
}
template <class T, class X>
inline T setbit(T a, X i) {
  T t = 1;
  return (a | (t << i));
}
template <class T, class X>
inline T resetbit(T a, X i) {
  T t = 1;
  return (a & (~(t << i)));
}
template <class T, class X>
inline T togglebit(T a, X i) {
  T t = 1;
  return (a ^ (t << i));
}
template <class T, class X, class Y>
inline T bigmod(T n, X m, Y mod) {
  unsigned long long ret = 1, a = n % mod;
  while (m) {
    if (m & 1) ret = (ret * a) % mod;
    m >>= 1;
    a = (a * a) % mod;
  }
  ret %= mod;
  return (T)ret;
}
template <class T, class Y>
inline T modinv(T n, Y mod) {
  return bigmod(n, mod - 2, mod);
}
int main() {
  ios_base::sync_with_stdio(false);
  int ff[100005], ss[100005];
  int n, x, ans = 0, tt, nn = 0;
  cin >> n >> x;
  for (int i = 0; i < n; i++) cin >> ff[i] >> ss[i];
  nn = 1;
  for (int i = 0; i < n; i++) {
    while (nn + x <= ff[i]) {
      nn += x;
    }
    ans += ss[i] - nn + 1;
    nn = ss[i] + 1;
  }
  cout << ans;
  return 0;
}
