#include <bits/stdc++.h>
using namespace std;
inline char read() {
  static const int IN_LEN = 1000000;
  static char buf[IN_LEN], *s, *t;
  return (s == t               ? t = (s = buf) + fread(buf, 1, IN_LEN, stdin),
          (s == t ? -1 : *s++) : *s++);
}
template <class T>
inline void read(T &x) {
  static bool iosig;
  static char c;
  for (iosig = false, c = read(); !isdigit(c); c = read()) {
    if (c == '-') iosig = true;
    if (c == -1) return;
  }
  for (x = 0; isdigit(c); c = read()) x = ((x + (x << 2)) << 1) + (c ^ '0');
  if (iosig) x = -x;
}
const int N = 300005, inf = 1e9 + 1;
int mx, n, k, id, a[N], b[N], len[N];
struct seg {
  int l, r, col;
  inline bool operator<(const seg &rhs) const {
    return l < rhs.l || (l == rhs.l && r < rhs.r);
  }
};
set<seg> s;
vector<seg> e[N];
inline void init() {
  s.insert((seg){0, inf, 0});
  for (int i = 1; i <= n; ++i) {
    auto it = s.lower_bound((seg){a[i], inf, 0});
    --it;
    if (it->r >= b[i]) {
      e[i].push_back((seg){a[i], b[i], it->col});
      if (it->l < a[i]) s.insert((seg){it->l, a[i] - 1, it->col});
      if (it->r > b[i]) s.insert((seg){b[i] + 1, it->r, it->col});
      s.erase(it);
      s.insert((seg){a[i], b[i], i});
      continue;
    }
    e[i].push_back((seg){a[i], it->r, it->col});
    if (it->l < a[i]) s.insert((seg){it->l, a[i] - 1, it->col});
    s.erase(it++);
    if (it->r < a[i]) ++it;
    while (it->r < b[i]) e[i].push_back(*it), s.erase(it++);
    e[i].push_back((seg){it->l, b[i], it->col});
    if (it->r > b[i]) s.insert((seg){b[i] + 1, it->r, it->col});
    s.erase(it);
    s.insert((seg){a[i], b[i], i});
  }
}
inline bool check(int x) {
  int sum = 0;
  long long cnt = 0;
  for (int i = 1, l = 1; i <= n; ++i) {
    sum += (len[i] = b[i] - a[i] + 1);
    for (auto j : e[i]) {
      len[j.col] -= j.r - j.l + 1;
      if (j.col && j.col >= l) sum -= j.r - j.l + 1;
    }
    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 i = 1, l = 1; i <= n; ++i) {
    sum += (len[i] = b[i] - a[i] + 1);
    for (auto j : e[i]) {
      len[j.col] -= j.r - j.l + 1;
      if (j.col && j.col >= l) sum -= j.r - j.l + 1;
      if (j.col && j.col < l) now -= (long long)j.col * (j.r - j.l + 1);
    }
    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)(cnt - k) * x;
}
int main() {
  read(n), read(k);
  for (int i = 1; i <= n; ++i) read(a[i]), read(b[i]), --b[i];
  init();
  int l = 1, r = 1e9, ans = 0;
  while (l <= r) {
    int mid = (l + r) / 2;
    if (check(mid))
      l = mid + 1, ans = mid;
    else
      r = mid - 1;
  }
  return printf("%lld", solve(ans)), 0;
}
