#include <bits/stdc++.h>
using namespace std;
set<pair<pair<int, int>, int> > cho;
set<pair<pair<int, int>, int> >::iterator split(int p) {
  set<pair<pair<int, int>, int> >::iterator it =
      cho.lower_bound(make_pair(make_pair(p + 1, 0), 0));
  it--;
  if (it->first.first == p) return it;
  cho.erase(it);
  pair<pair<int, int>, int> stl = make_pair(make_pair(it->first.first, p),
                                            it->second),
                            str = make_pair(make_pair(p, it->first.second),
                                            it->second);
  cho.insert(stl);
  return cho.insert(str).first;
}
vector<pair<int, int> > op[300005];
void Mdf(int l, int r, int i) {
  set<pair<pair<int, int>, int> >::iterator tr = split(r), tl = split(l);
  op[i].emplace_back(i, r - l);
  for (set<pair<pair<int, int>, int> >::iterator p = tl; p != tr; p++)
    op[i].emplace_back(p->second, p->first.first - p->first.second);
  cho.erase(tl, tr);
  cho.insert(make_pair(make_pair(l, r), i));
}
long long ans = 0, tr[300005];
int n, k;
bool chk(int len) {
  long long sm = 0, sum = 0, tot = 0, cur = 0;
  for (int i = 0; i <= n; i++) tr[i] = 0;
  for (int r = 1, l = 1; r <= n; r++) {
    for (auto x : op[r]) {
      tr[x.first] -= x.second;
      if (x.first < l)
        sum -= 1ll * (l - x.first - 1) * x.second, cur -= x.second;
    }
    while (cur >= len) sum += cur, cur += tr[l], l++;
    sm += sum;
    tot += l - 1;
  }
  if (tot < k) return false;
  ans = sm - 1ll * (tot - k) * len;
  return true;
}
int main() {
  scanf("%d%d", &n, &k);
  cho.insert(make_pair(make_pair(1, 1000000000), 0));
  for (int i = 1, l, r; i <= n; i++) scanf("%d%d", &l, &r), Mdf(l, r, i);
  int l = 1, r = 1000000000;
  for (int mid; l <= r;)
    if (chk(mid = ((l + r) >> 1)))
      l = mid + 1;
    else
      r = mid - 1;
  printf("%lld", ans);
}
