import java.io.PrintWriter
import java.util.*
import kotlin.math.*

fun PrintWriter.solve() {
    val n = nextInt()
    val d = nextInt()
    val (p, q) = Array(n) { nextInt() to nextInt() }.unzip().toList().map { it.toTypedArray() }
    var ans = Long.MIN_VALUE
    for (i in 0 until n) {
        for (j in 0 until n) {
            if (i == j) continue
            var now = 0L
            var min = 0L
            for (k in 0 until d) {
                if (k and 1 == 0) {
                    now -= p[i]
                    min = minOf(min, now)
                    now += q[i]
                } else {
                    now -= p[j]
                    min = minOf(min, now)
                    now += q[j]
                }
            }
            ans = maxOf(ans, min)
        }
    }
    println(ans)
}

fun main() {
    Thread(null, {
        val writer = PrintWriter(System.out, false)
        writer.solve()
        writer.flush()
    }, "solve", abs(1L.shl(26)))
        .apply { setUncaughtExceptionHandler { _, e -> e.printStackTrace(); kotlin.system.exitProcess(1) } }
        .apply { start() }.join()
}

// region Scanner
private var st = StringTokenizer("")
private val br = System.`in`.bufferedReader()

fun next(): String {
    while (!st.hasMoreTokens()) st = StringTokenizer(br.readLine())
    return st.nextToken()
}

fun nextInt() = next().toInt()
fun nextLong() = next().toLong()
fun nextLine() = br.readLine()
fun nextDouble() = next().toDouble()
// endregion