#include <iostream>

#include <cstdio>

#include <algorithm>

#include <cmath>

#include <cstring>

#include <string>

using namespace std;

#define LL long long

#define gc getchar()

inline int read() {int x = ; char c = gc; while(c < '' || c > '') c = gc;

while(c >= '' && c <= '') x = x *  + c - '', c = gc; return x;}

inline LL read_LL() {LL x = ; char c = gc; while(c < '' || c > '') c = gc;

while(c >= '' && c <= '') x = x *  + c - '', c = gc; return x;}

#undef gc

const int N = 1e6 + ;

struct Node {int l, r, len;} G[N >> ];

int A[N];

int Max[N << ], W[N << ], F[N << ];

int n, m;

inline bool Cmp(Node a, Node b) {return a.len > b.len;}

#define lson jd << 1

#define rson jd << 1 | 1

void Push_down(int jd) {

    F[lson] += F[jd], F[rson] += F[jd];

    Max[lson] += F[jd], Max[rson] += F[jd];

    F[jd] = ;

}

void Sec_G(int l, int r, int jd, int x, int y, int num) {

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

        Max[jd] += num;

        F[jd] += num;

        return ;

    }

    if(F[jd]) Push_down(jd);

    int mid = (l + r) >> ;

    if(x <= mid) Sec_G(l, mid, lson, x, y, num);

    if(y > mid ) Sec_G(mid + , r, rson, x, y, num);

    Max[jd] = max(Max[lson], Max[rson]);

}

bool use[N];

int main() {

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

    int tot = ;

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

        G[i].l = read(), G[i].r = read(), G[i].len = G[i].r - G[i].l; A[++ tot] = G[i].l, A[++ tot] = G[i].r;

    }

    sort(G + , G + n + , Cmp);

    sort(A + , A + tot + );

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

        G[i].l = lower_bound(A + , A + tot + , G[i].l) - A;

        G[i].r = lower_bound(A + , A + tot + , G[i].r) - A;

    }

    int R = , Answer = (int)1e9 + ;

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

        if(R == n) break;

        if(use[i] == ) {

            Sec_G(, tot, , G[i].l, G[i].r, );

            use[i] = ;

        }

        while(Max[] < m && R < n) {

            R ++;

            Sec_G(, tot, , G[R].l, G[R].r, );

            use[R] = ;

            if(Max[] >= m) {

                Answer = min(Answer, G[i].len - G[R].len);

                break;

            }

        }

        if(use[i]) {

            Sec_G(, tot, , G[i].l, G[i].r, -);

            use[i] = ;

        }

    }

    if(Answer == (int)1e9 + ) cout << "-1";

    else cout << Answer;

    return ;

}