#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, f, i, j;
  cin >> n >> f;
  long long ans(0), k, l, mini;
  vector<pair<long long, long long> > v, v1;
  for (i = 0; i < n; i++) {
    cin >> k >> l;
    v.push_back(make_pair(k, l));
  }
  for (i = 0; i < n; i++) {
    v1.push_back(make_pair(
        min(2 * v[i].first, v[i].second) - min(v[i].first, v[i].second), i));
  }
  sort(v1.begin(), v1.end(), greater<pair<long long, long long> >());
  for (i = 0; i < f; i++) {
    mini = min(2 * v[v1[i].second].first, v[v1[i].second].second);
    ans += mini;
    v[v1[i].second].first = -1;
  }
  for (i = 0; i < n; i++) {
    if (v[i].first != -1) ans += min(v[i].first, v[i].second);
  }
  cout << ans;
  return 0;
}
