import java.beans.Customizer;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.StringTokenizer;

public class CF415B {
	static class Day implements Comparable<Day> {

		int idx;
		int products;

		@Override
		public int compareTo(Day arg0) {
			return products - arg0.products;
		}

		public Day(int idx, int products) {
			this.idx = idx;
			this.products = products;
		}

	}

	public static void main(String[] args) {
		MyScanner reader = new MyScanner();
		StringBuilder sb = new StringBuilder();
		// int a = reader.nextInt();
		// int b = reader.nextInt();

		int n = reader.nextInt();
		int f = reader.nextInt();

		List<Day> arr = new ArrayList<>();
		int[] customers = new int[n];
		int[] products = new int[n];
		for (int i = 0; i < n; i++) {
			products[i] = reader.nextInt();
			customers[i] = reader.nextInt();
			if (customers[i] <= 2 * products[i]) {
				arr.add(new Day(i, customers[i]));
			} else {
				arr.add(new Day(i, 2 * products[i]));
			}

		}

		long sum = 0;
		Collections.sort(arr);
		boolean[] used = new boolean[n];
		// System.out.println(arr);
		for (int i = arr.size() - 1; i >= arr.size() - f; i--) {
			sum += arr.get(i).products;
			used[arr.get(i).idx] = true;
		}
		for (int i = 0; i < used.length; i++) {
			if (used[i] == false) {
				if (customers[i] <= products[i]) {
					sum += customers[i];
				} else {
					sum += products[i];
				}
			}
		}
		System.out.println(sum);
	}

	public static class MyScanner {
		BufferedReader br;
		StringTokenizer st;

		public MyScanner() {
			br = new BufferedReader(new InputStreamReader(System.in));
		}

		String next() {
			while (st == null || !st.hasMoreElements()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}

		int nextInt() {
			return Integer.parseInt(next());
		}
	}

}