#include <bits/stdc++.h>
namespace IO {
inline char gc() {
  static const int Rlen = 1 << 22 | 1;
  static char buf[Rlen], *p1, *p2;
  return (p1 == p2) && (p2 = (p1 = buf) + fread(buf, 1, Rlen, stdin), p1 == p2)
             ? EOF
             : *p1++;
}
template <typename T>
inline T get() {
  char c;
  T num;
  while (!isdigit(c = gc()))
    ;
  num = c ^ 48;
  while (isdigit(c = gc())) num = (num + (num << 2) << 1) + (c ^ 48);
  return num;
}
inline int gi() { return get<int>(); }
}  // namespace IO
using namespace IO;
using std::cerr;
using std::cout;
using pii = std::pair<int, int>;
const int N = 3e5 + 7, INF = 0x3f3f3f3f;
int n, K;
int l[N], r[N], len[N];
struct seg {
  int l, r, tim;
};
bool operator<(const seg &a, const seg &b) {
  return a.l < b.l || (a.l == b.l && a.r < b.r);
}
std::set<seg> s;
std::vector<pii> v[N];
inline void init() {
  s.insert((seg){0, INF, 0});
  for (int register i = 1; i <= n; ++i) {
    auto it = --s.lower_bound((seg){l[i], INF, 0});
    if (it->r >= r[i]) {
      v[i].push_back(pii(r[i] - l[i] + 1, it->tim));
      if (it->l < l[i]) s.insert((seg){it->l, l[i] - 1, it->tim});
      if (it->r > r[i]) s.insert((seg){r[i] + 1, it->r, it->tim});
      s.erase(it);
      s.insert((seg){l[i], r[i], i});
    } else {
      v[i].push_back(pii(it->r - l[i] + 1, it->tim));
      if (it->l < l[i]) s.insert((seg){it->l, l[i] - 1, it->tim});
      s.erase(it++);
      if (it->r < l[i]) ++it;
      while (it->r < r[i])
        v[i].push_back(pii(it->r - it->l + 1, it->tim)), s.erase(it++);
      v[i].push_back(pii(r[i] - it->l + 1, it->tim));
      if (it->r > r[i]) s.insert((seg){r[i] + 1, it->r, it->tim});
      s.erase(it);
      s.insert((seg){l[i], r[i], i});
    }
  }
}
inline bool check(int x) {
  int sum = 0;
  long long cnt = 0;
  for (int register i = 1, L = 1; i <= n; ++i) {
    sum += len[i] = r[i] - l[i] + 1;
    for (auto t : v[i]) {
      len[t.second] -= t.first;
      if (t.second >= L) sum -= t.first;
    }
    while (sum >= x) sum -= len[L], ++L;
    cnt += L - 1;
  }
  return cnt >= K;
}
inline long long solve(int x) {
  int sum = 0;
  long long ans = 0, cnt = 0, now = 0;
  for (int register i = 1, L = 1; i <= n; ++i) {
    sum += len[i] = r[i] - l[i] + 1;
    for (auto t : v[i]) {
      len[t.second] -= t.first;
      if (t.second >= L) sum -= t.first;
      if (t.second && t.second < L) now -= (long long)t.second * t.first;
    }
    while (sum >= x) sum -= len[L], now += (long long)L * len[L], ++L;
    ans += now + (long long)sum * (L - 1);
    cnt += L - 1;
  }
  return ans - (long long)x * (cnt - K);
}
signed main() {
  n = gi(), K = gi();
  for (int register i = 1; i <= n; ++i) l[i] = gi(), r[i] = gi() - 1;
  init();
  int l = 1, r = INF, ans = 0, mid;
  while (l <= r)
    check(mid = l + r >> 1) ? (ans = mid, l = mid + 1) : (r = mid - 1);
  cout << solve(ans) << "\n";
  return 0;
}
