#include <bits/stdc++.h>
using namespace std;
long long n, f, a, b;
vector<long long> V;
long long ans;
int main() {
  scanf("%lld %lld", &n, &f);
  for (int i = 1; i <= n; i++) {
    scanf("%lld %lld", &a, &b);
    if (a >= b) {
      V.push_back(0);
      ans += b;
    } else {
      if (a * 2 <= b) {
        V.push_back(a);
      } else {
        V.push_back(b - a);
      }
      ans += a;
    }
  }
  sort(V.begin(), V.end());
  reverse(V.begin(), V.end());
  for (int i = 0; i < f; i++) {
    ans += V[i];
  }
  cout << ans << endl;
  return 0;
}
