#include<cstdio>

 #include<cstring>

 #include<algorithm>

 using namespace std;

 const int maxn=;

 inline int read(){

     char c=getchar();int a=;

     while(c<''||c>'')  c=getchar();

     while(c>=''&&c<=''){a=a*+c-'';c=getchar();}

     return a;

 }

 struct ST{

     int x,id;bool isl;

 }nd[maxn*];

 struct Line{

     int l,r,siz;

 }ln[maxn];

 struct Node{

     int l,r,num,laz;

 }tr[maxn*];

 int M,N;

 inline bool cmp1(ST a,ST b){return a.x<b.x;}

 inline bool cmp2(Line a,Line b){return a.siz<b.siz;}

 void build(int i,int l,int r){

     tr[i].l=l;tr[i].r=r;tr[i].num=tr[i].laz=;

     if(l>=r)  return;

     int m=(l+r)>>;build(i<<,l,m);build(+(i<<),m+,r);

 }

 inline void pushdown(int i){

     if(!tr[i].laz)  return;

     if(tr[i].l==tr[i].r){tr[i].laz=;return;}

     int l=i<<,r=(i<<)+;

     tr[l].num+=tr[i].laz;tr[r].num+=tr[i].laz;

     tr[l].laz+=tr[i].laz;tr[r].laz+=tr[i].laz;

     tr[i].laz=;return;

 }

 inline void update(int i){if(tr[i].l<tr[i].r)tr[i].num=max(tr[i<<].num,tr[(i<<)+].num);}

 void add(int i,int l,int r,int x){

     if(tr[i].l==l&&tr[i].r==r){tr[i].num+=x;tr[i].laz+=x;return;}

     int m=(tr[i].l+tr[i].r)>>;pushdown(i);

     if(l<=m)  add(i<<,l,min(m,r),x);

     if(r>m)  add((i<<)+,max(l,m+),r,x);

     update(i);

 }

 int main(){

     int i,j,k,ans=0x7fffffff;

     N=read();M=read();

     for(i=;i<=N;i++){

         ln[i].l=read();ln[i].r=read();ln[i].siz=ln[i].r-ln[i].l;

         nd[i].x=ln[i].l;nd[i].id=i;nd[i].isl=;

         nd[i+N].x=ln[i].r;nd[i+N].id=i;nd[i+N].isl=;

     }sort(nd+,nd+N*+,cmp1);

     for(i=,j=;i<=N*;i++){

         if(nd[i].x!=nd[i-].x)  j++;

         if(nd[i].isl) ln[nd[i].id].l=j;

         else  ln[nd[i].id].r=j;

     }sort(ln+,ln+N+,cmp2);

     build(,,N*);

     for(i=,j=;i<=N;i++){

         add(,ln[i].l,ln[i].r,);

         if(tr[].num>=M){

             while(tr[].num>=M){j++;add(,ln[j].l,ln[j].r,-);}

             ans=min(ans,ln[i].siz-ln[j].siz);

         }

     }

     if(ans!=0x7fffffff) printf("%d",ans);

     else  printf("-1");

 }