#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001

int main(){
	
	int N,K;
	cin>>N>>K;
	
	vector<pair<int,int>> lr(N);
	rep(i,N){
		cin>>lr[i].first>>lr[i].second;
	}
	sort(lr.begin(),lr.end());
	
	{
		vector<pair<int,int>> nl;
		rep(i,N){
			while(nl.size()>0 && nl.back().second>=lr[i].second)nl.pop_back();
			nl.push_back(lr[i]);
		}
		swap(lr,nl);
		N = lr.size();
	}
	
	vector nxt(N,vector<int>(18,N));
	rep(i,N){
		int d = distance(lr.begin(),lower_bound(lr.begin(),lr.end(),make_pair(lr[i].second,-1)));
		nxt[i][0] = d;
	}
	for(int i=1;i<18;i++){
		rep(j,N){
			if(nxt[j][i-1]==N)nxt[j][i] = N;
			else nxt[j][i] = nxt[nxt[j][i-1]][i-1];
		}
	}
	
	int ans = Inf32;
	K--;
	rep(i,N){
		//cout<<lr[i].first<<' '<<lr[i].second<<endl;
		int cur = i;
		rep(j,18){
			if((K>>j)&1){
				if(cur!=N)cur = nxt[cur][j];
			}
		}
		if(cur!=N){
			ans = min(ans,lr[cur].second - lr[i].first);
		}
	}
	if(ans==Inf32)ans = -1;
	cout<<ans<<endl;
	
	return 0;
}