#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, f, a, b;
  vector<pair<long long int, long long int> > v;
  cin >> n >> f;
  int i;
  for (i = 0; i < n; i++) {
    cin >> a >> b;
    v.push_back(make_pair(a, b));
  }
  long long int ans = 0;
  vector<long long int> t;
  for (i = 0; i < n; i++) {
    ans = ans + min(v[i].first, v[i].second);
    long long int mins =
        min(v[i].first * 2, v[i].second) - min(v[i].first, v[i].second);
    t.push_back(mins);
  }
  sort(t.begin(), t.end());
  for (i = n - 1; i >= 0; i--) {
    if (t[i] > 0 && f != 0) {
      ans = ans + t[i];
      f--;
    }
    if (f == 0) break;
  }
  cout << ans;
}
