#include <bits/stdc++.h>
#define lson p<<1
#define rson (p<<1)|1
using namespace std;
inline int gi () {int x=0, w=0; char ch=0;while (! (ch>='0' && ch<='9') ) {if (ch=='-') w=1;ch=getchar ();}while (ch>='0' && ch<='9') {x= (x<<3) + (x<<1) +  (ch^48);ch=getchar ();}return w?-x:x;
}const int N=5e5+10;
int n,m,head=1,Ans=0x3f3f3f3f,Num,Ls[N<<1],LS[N<<1],Cnt[N<<3],Tag[N<<3];struct Sect {int l, r, val;
}s[N];
bool Cmp (Sect a, Sect b) {return a.val<b.val;
}void Pushup (int p) {Cnt[p]=max (Cnt[lson], Cnt[rson]);
}void Pushdown (int p) {if (!Tag[p]) return;Tag[lson]+=Tag[p];Tag[rson]+=Tag[p];Cnt[lson]+=Tag[p];Cnt[rson]+=Tag[p];Tag[p]=0;
}void Modify (int p, int l, int r, int Ml, int Mr, int val) {if (l>=Ml && Mr>=r) {Cnt[p]+=val;Tag[p]+=val;return;}Pushdown (p);int Mid= (l+r) >> 1;if (Ml<=Mid) Modify (lson, l, Mid, Ml, Mr, val);if (Mr>Mid) Modify (rson, Mid+1, r, Ml, Mr, val);Pushup (p);
}int main ()
{n=gi (), m=gi ();memset (Ls, -1, sizeof (Ls) );memset (LS, -1, sizeof (LS) );for (int i=1;i<=n;++i) {s[i].l=gi (), s[i].r=gi ();s[i].val=s[i].r-s[i].l;Ls[++Num]=s[i].l, Ls[++Num]=s[i].r;}sort (Ls+1, Ls+Num+1);for (int i=1, j=0;i<=Num;++i) {if (Ls[i]!=Ls[i-1]) LS[++j]=Ls[i];if (i==Num) {Num=j;break;}}for (int i=1;i<=n;++i) {s[i].l=lower_bound (LS+1, LS+Num+1, s[i].l) -LS;s[i].r=lower_bound (LS+1, LS+Num+1, s[i].r) -LS;}    sort (s+1, s+n+1, Cmp);for (int i=1;i<=n;++i) {Modify (1, 1, Num, s[i].l, s[i].r, 1);            while (Cnt[1]==m) {Ans=min (Ans, s[i].val-s[head].val);Modify (1, 1, Num, s[head].l, s[head].r, -1);head++;}}printf ("%d\n", Ans==0x3f3f3f3f?-1:Ans);return 0;
}