//minamoto

 #include<iostream>

 #include<cstdio>

 #include<algorithm>

 #define inf 0x3f3f3f3f

 using namespace std;

 #define getc() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)

 char buf[<<],*p1=buf,*p2=buf;

 template<class T>inline bool cmax(T&a,const T&b){return a<b?a=b,:;}

 template<class T>inline bool cmin(T&a,const T&b){return a>b?a=b,:;}

 inline int read(){

     #define num ch-'0'

     char ch;bool flag=;int res;

     while(!isdigit(ch=getc()))

     (ch=='-')&&(flag=true);

     for(res=num;isdigit(ch=getc());res=res*+num);

     (flag)&&(res=-res);

     #undef num

     return res;

 }

 const int N=5e5+;

 int sum[N<<],add[N<<],L[N],R[N],val[N<<],n,m,lim,cnt,ans=inf;

 struct node{

     int len,id;

     node(){}

     node(int len,int id):len(len),id(id){}

     inline bool operator <(const node &b)const

     {return len<b.len;}

 }a[N];

 inline void upd(int p){

     if(add[p]){

         add[p<<]+=add[p],add[p<<|]+=add[p];

         sum[p<<]+=add[p],sum[p<<|]+=add[p];

         add[p]=;

     }

 }

 void update(int p,int l,int r,int ql,int qr,int val){

     if(ql>r||qr<l) return;

     if(ql<=l&&qr>=r) return (void)(add[p]+=val,sum[p]+=val);

     int mid=(l+r)>>;upd(p);

     update(p<<,l,mid,ql,qr,val);

     update(p<<|,mid+,r,ql,qr,val);

     sum[p]=max(sum[p<<],sum[p<<|]);

 }

 int main(){

 //    freopen("testdata.in","r",stdin);

     n=read(),m=read();

     for(int i=;i<=n;++i)

     L[i]=val[++cnt]=read(),R[i]=val[++cnt]=read(),a[i]=node(R[i]-L[i],i);

     sort(a+,a++n);

     sort(val+,val++cnt),lim=unique(val+,val++cnt)-val-;

     for(int i=;i<=n;++i)

     L[i]=lower_bound(val+,val++lim,L[i])-val,R[i]=lower_bound(val+,val++lim,R[i])-val;

     int l=,r=;

     while(true){

         while(sum[]<m&&r<n){

             int i=a[++r].id,u=L[i],v=R[i];

             update(,,lim,u,v,);

         }

         if(sum[]<m) break;

         while(sum[]>=m&&l<n){

             int i=a[++l].id,u=L[i],v=R[i];

             update(,,lim,u,v,-);

         }

         cmin(ans,a[r].len-a[l].len);

     }

     printf("%d\n",ans==inf?-:ans);

     return ;

 }