import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }

string COLOR(string s = "") { return "\x1b[" ~ s ~ "m"; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 1001001001;
enum E = 18;

void main() {
  try {
    for (; ; ) {
      const N = readInt;
      const K = readInt;
      alias LR = Tuple!(int, "l", int, "r");
      auto A = new LR[N];
      foreach (i; 0 .. N) {
        A[i].l = readInt;
        A[i].r = readInt;
      }
      A.sort!"a.r < b.r";
      
      auto nxt = new int[][](E, N + 1);
      {
        auto js = iota(N).array;
        js.sort!((j0, j1) => (A[j0].l > A[j1].l));
        int pos;
        int jm = N;
        foreach_reverse (i; 0 .. N) {
          for (; pos < N && A[i].r <= A[js[pos]].l; ++pos) {
            chmin(jm, js[pos]);
          }
          nxt[0][i] = jm;
        }
      }
      nxt[0][N] = N;
      debug {
        writeln("A = ", A);
        writeln("nxt[0] = ", nxt[0]);
      }
      foreach (e; 0 .. E - 1) {
        foreach (i; 0 .. N + 1) {
          nxt[e + 1][i] = nxt[e][nxt[e][i]];
        }
      }
      
      int ans = INF;
      foreach (i; 0 .. N) {
        int j = i;
        foreach_reverse (e; 0 .. E) if ((K - 1) >> e & 1) {
          j = nxt[e][j];
        }
        if (j < N) {
          chmin(ans, A[j].r - A[i].l);
        }
      }
      writeln((ans < INF) ? ans : -1);
    }
  } catch (EOFException e) {
  }
}