#include<algorithm>

#include<iostream>

#include<cstdlib>

#include<cstring>

#include<cstdio>

#include<cmath>

using namespace std;

typedef long long LL;

#define INF 0x7fffffff

#define N 5000010

int n,m;

struct data

{

    int l,r,k;

}a[N];

bool operator < (data x,data y)

{

    return x.k<y.k;

}

int b[N];

int add[N],sum[N];

int ans=INF,cnt;

inline int getint()

{

    int x=0,f=1;

    char ch=getchar();

    while (ch>'9' || ch<'0')

    {

        if (ch=='-')

            f=-1;

        ch=getchar();

    }

    while (ch>='0' && ch<='9')

    {

        x=x*10+ch-'0';

        ch=getchar();

    }

    return x*f;

}

inline void pushup(int now)

{

    sum[now]=max(sum[now<<1],sum[now<<1|1]);

}

inline void pushdown(int now)

{

    if (add[now])

    {

        sum[now<<1]+=add[now];

        sum[now<<1|1]+=add[now];

        add[now<<1]+=add[now];

        add[now<<1|1]+=add[now];

        add[now]=0;

    }

}

inline void update(int nowl,int nowr,int now,int l,int r,int d)

{

    if (nowl>=l && nowr<=r)

    {

        sum[now]+=d;

        add[now]+=d;

        return ;

    }

    pushdown(now);

    int mid=nowl+nowr>>1;

    if (l<=mid)

        update(nowl,mid,now<<1,l,r,d);

    if (r>mid)

        update(mid+1,nowr,now<<1|1,l,r,d);

    pushup(now);

}

inline int get(int x)

{

    int l=1,r=cnt,mid;

    while (l<r)

    {

        mid=l+r>>1;

        if (b[mid]>=x)

            r=mid;

        else

            l=mid+1;

    }

    return l;

}

int main()

{

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

    for (int i=1;i<=n;i++)

    {

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

        a[i].k=a[i].r-a[i].l+1;

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

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

    }

    sort(a+1,a+n+1);

    sort(b+1,b+cnt+1);

    for (int i=1;i<=n;i++)

        a[i].l=get(a[i].l),a[i].r=get(a[i].r);

    for (int i=1,j=0;j<=n;)

    {

        if (sum[1]>m)

        {

            update(1,cnt,1,a[i].l,a[i].r,-1);

            i++;

        }

        else if (sum[1]<m)

        {

            j++;

            if (j<=n)

                update(1,cnt,1,a[j].l,a[j].r,1);

        }

        else

        {

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

            update(1,cnt,1,a[i].l,a[i].r,-1);

            i++;

        }

    }

    printf("%d\n",ans!=INF ? ans : -1);

    return 0;

}