#include <iostream>

#include <cstdio>

#include <algorithm>

using namespace std;

int n,m,b[],c[],d[],mx[],add[],l=,r=,ans=0x7fffffff,num;

struct seg

{

    int x,y,len;

    inline bool operator <(const seg& rhs) const

    {

        return len>rhs.len;

    }

}a[];

inline int read()

{

    char ch=getchar();int x=;

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

    while(ch<='' && ch>='')x=(x<<)+(x<<)+(ch^''),ch=getchar();

    return x;

}

inline void ins(int l,int r,int x,int y,int k,int v)

{

    if (x<=l&&y>=r)

    {

        add[k]+=v;

        mx[k]+=v;

        return;

    }

    int mid=l+r>>;

    if (x<=mid) ins(l,mid,x,y,k<<,v);

    if (y>mid) ins(mid+,r,x,y,k<<|,v);

    mx[k]=max(mx[k<<],mx[k<<|]);

    mx[k]+=add[k];

}

int low(int x)

{

    int l=,r=num,mid;

    while (l<r)

    {

        mid=(l+r)>>;

        if (b[mid]>=x) r=mid;

        else l=mid+;

    }

    return l;

}

int main()

{

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

    num=;

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

    {

        a[i].x=read();

        a[i].y=read();

        a[i].len=a[i].y-a[i].x;

        b[++num]=a[i].x;

        b[++num]=a[i].y;

    }

    sort(a+,a+n+);

    sort(b+,b+num+);

    for (int i=;i<=n;i++) a[i].x=low(a[i].x),a[i].y=low(a[i].y);

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

    {

        ins(,num,a[r].x,a[r].y,,);

        while (mx[]>=m)

        {

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

            ins(,num,a[l].x,a[l].y,,-);

            l++;

        }

    }

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

    else printf("-1");

    return ;

}