#include <bits/stdc++.h>
using namespace std;
set<pair<int, int> > s;
set<pair<int, int> >::iterator t;
vector<pair<int, int> > v[303030];
int l[303030], r[303030];
int h[303030];
int n;
long long cnt, ans;
void work(int lim) {
  int v1, v2;
  long long sum = 0;
  cnt = ans = 0;
  sum = 0;
  v1 = v2 = 0;
  memset(h, 0, sizeof(h));
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < v[i].size(); j++) {
      if (v[i][j].first <= v1) {
        v2 += v[i][j].second;
        sum += (long long)v[i][j].second * (v1 - v[i][j].first + 1);
      }
      h[v[i][j].first] += v[i][j].second;
    }
    while (v1 < n && v2 + h[v1 + 1] >= lim) {
      v1++;
      v2 += h[v1];
      sum += v2;
    }
    ans += sum;
    cnt += v1;
  }
}
int main() {
  int k, la, la1;
  scanf("%d%d", &n, &k);
  s.clear();
  s.insert(make_pair(0, 0));
  s.insert(make_pair(1000000000, 0));
  for (int i = 1; i <= n; i++) {
    v[i].clear();
    scanf("%d%d", &l[i], &r[i]);
    t = (--s.lower_bound(make_pair(l[i], n)));
    la = t->second;
    if (t->first == l[i])
      s.erase(t++);
    else
      t++;
    la1 = l[i];
    while (true) {
      v[i].push_back(make_pair(la + 1, min(r[i], t->first) - la1));
      if (r[i] > t->first) {
        la = t->second;
        la1 = t->first;
        s.erase(t++);
      } else
        break;
    }
    v[i].push_back(make_pair(i + 1, l[i] - r[i]));
    s.insert(make_pair(l[i], i));
    if (r[i] < t->first) s.insert(make_pair(r[i], la));
  }
  int l1 = 1, r1 = 1000000000, mid;
  while (l1 < r1) {
    mid = (l1 + r1 + 1) >> 1;
    work(mid);
    if (cnt >= k)
      l1 = mid;
    else
      r1 = mid - 1;
  }
  work(l1);
  printf("%lld\n", ans - (cnt - k) * l1);
  return 0;
}
