import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.bigint, std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static File _f;
void file_io(string fn) { _f = File(fn, "r"); }
static string[] s_rd;
T _RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T _RD(T = long)(File f) { while(!s_rd.length) s_rd = f.readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T[] _RDA(T = long)(T fix = 0) { auto r = readln.chomp.split.to!(T[]); r[] += fix; return r; }
T[] _RDA(T = long)(File f, T fix = 0) { auto r = f.readln.chomp.split.to!(T[]); r[] += fix; return r; }
T RD(T = long)() { if (_f.isOpen) return _RD!T(_f); else return _RD!T; }
T[] RDA(T = long)(T fix = 0) { if (_f.isOpen)  return _RDA!T(_f, fix); else return _RDA!T(fix); }

size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

void chmin(T)(ref T x, T y) { x = min(x, y); } void chmax(T)(ref T x, T y) { x = max(x, y); }
bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
T lcm(T)(T x, T y) { return x * (y / gcd(x, y)); }
double[] rotate(double[] vec, double rad) { return [cos(rad)*vec[0] - sin(rad)*vec[1], sin(rad)*vec[0] + cos(rad)*vec[1]]; }

//long mod = 10^^9 + 7;
long mod = 998_244_353;
//long mod = 1_000_003;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }
void modpow(ref long x, long y) { if (!y) { x = 1; return; } auto t = x; x.modpow(y>>1); x.modm(x); if (y&1) x.modm(t); }
void modd(ref long x, long y) { y.modpow(mod - 2); x.modm(y); }

T binarySearch(alias pred, T)(T ok, T ng)
{ 
	while (abs(ok-ng) > 1)
	{
		auto mid = (ok+ng)/2;
		if (unaryFun!pred(mid)) ok = mid;
		else ng = mid;
	}
	return ok;
}

void main()
{
	auto N = RD;
	auto D = RD;
	auto P = new long[](N);
	auto Q = new long[](N);
	foreach (i; 0..N)
	{
		P[i] = RD;
		Q[i] = RD;
	}

	auto xyz = new long[][][](N, N);
	foreach (i; 0..N)
	{
		foreach (j; 0..N)
		{
			if (i == j) continue;
			long cnt;
			cnt -= P[i];
			auto x = cnt;
			cnt += Q[i];
			cnt -= P[j];
			auto y = cnt;
			cnt += P[j];
			auto z = cnt;
			xyz[i][j] = [x, y, z];
		}
	}

	long ans = long.min;
	if (D == 1)
		ans = -(P.minElement);
	else if (D == 2)
	{
		foreach (i; 0..N)
		{
			foreach (j; 0..N)
			{
				if (i == j) continue;
				ans.chmax(min(xyz[i][j][0], xyz[i][j][1]));
			}
		}
	}
	else
	{
		foreach (i; 0..N)
		{
			foreach (j; 0..N)
			{
				if (i == j) continue;
				if (xyz[i][j][2] < 0) continue;
				ans.chmax(min(xyz[i][j][0], xyz[i][j][1]));
			}
		}

		long[][] list;
		foreach (i; 0..N)
		{
			auto z = Q[i] - P[i];
			list ~= [z, P[i], i];
		}

		list.sort!"a[0] == b[0] ? a[1] < b[1] : a[0] > b[1]";
		bool f(long x)
		{
			long pos, last = -1;
			long ii, jj;
			foreach (i; 0..D)
			{
				auto rem = x - pos;
				while (ii < list.length)
				{
					if (list[ii][1] <= rem) break;
					++ii;
				}
				if (ii == list.length) return false;

				long kk;
				if (last == list[ii][2])
				{
					while (jj < list.length)
					{
						if (list[jj][1] <= rem) break;
						++jj;
					}
					if (jj == list.length) return false;

					kk = jj;
				}
				pos -= list[kk][0];
				last = kk;
			}
			return true;
		}

		ans.chmax(-binarySearch!(f)(10L^^9, 0L));
	}

	writeln(ans);
	stdout.flush;
	debug readln;
}