#include <bits/stdc++.h>
using namespace std;
int q[123456];
vector<int> sq;
int main() {
  int n, k;
  int a, b;
  scanf("%d%d", &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%d%d", &a, &b);
    sq.push_back(min(2 * a, b) - min(a, b));
    q[i] = min(a, b);
    1;
  }
  sort(sq.begin(), sq.end(), greater<int>());
  long long sum = 0;
  int flag = 1;
  for (int i = 0; i < k; i++) sum += sq[i], sq[i] < 0 ? flag = 0 : 0;
  for (int i = 1; i <= n; i++) sum += q[i];
  printf("%lld", flag * sum);
}
