#include<cstdio>

 #include<iostream>

 #include<algorithm>

 #define N 1000009

 #define INF 2147483647

 using namespace std;

 int n,m,i,j,cnt,ans=INF,sum[N<<],lazy[N<<],b[N<<];

 struct node{ int l,r,len; } a[N];

 bool cmp(const node &x,const node &y)

 {

     return x.len<y.len;

 }

 int find(int l,int r,int x)

 {

     if (l==r) return l;

     int mid=l+r>>;

     if (x<=b[mid]) find(l,mid,x);

     else find(mid+,r,x);

 }

 void up_date(int k,int x)

 {

     sum[k]+=x,lazy[k]+=x;

 }

 void change(int cur,int L,int R,int l,int r,int val)

 {

     if (L==l && R==r) { up_date(cur,val); return; }

     int mid=L+R>>;

     if (lazy[cur])

     {

         up_date(cur<<,lazy[cur]);

         up_date(cur<<|,lazy[cur]);

         lazy[cur]=;

     }

     if (r<=mid) change(cur<<,L,mid,l,r,val);

     else if (l>mid) change(cur<<|,mid+,R,l,r,val);

          else change(cur<<,L,mid,l,mid,val),change(cur<<|,mid+,R,mid+,r,val);

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

 }

 void solve()

 {

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

     {

         while (sum[]<m)

         {

             if (j==n) return; j++;

             change(,,cnt,a[j].l,a[j].r,);

         }

         ans=min(ans,a[j].len-a[i].len);

         change(,,cnt,a[i].l,a[i].r,-);

     }

 }

 int main()

 {

     scanf("%d%d",&n,&m);

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

     {

         scanf("%d%d",&a[i].l,&a[i].r);

         b[++cnt]=a[i].l,b[++cnt]=a[i].r;

         a[i].len=a[i].r-a[i].l;

     }

     sort(a+,a+n+,cmp),sort(b+,b+cnt+);

     for (i=;i<=n;i++) a[i].l=find(,cnt,a[i].l),a[i].r=find(,cnt,a[i].r);

     solve(); printf("%d\n",ans<INF?ans:-);

     return ;

 }