#include <bits/stdc++.h>
#define GET_MACRO(_1,_2,_3,_4,_5,_6,_7,_8,NAME,...) NAME
#define pr(...) cerr<< GET_MACRO(__VA_ARGS__,pr8,pr7,pr6,pr5,pr4,pr3,pr2,pr1)(__VA_ARGS__) <<endl
#define pr1(a) (#a)<<"="<<(a)<<" "
#define pr2(a,b) pr1(a)<<pr1(b)
#define pr3(a,b,c) pr1(a)<<pr2(b,c)
#define pr4(a,b,c,d) pr1(a)<<pr3(b,c,d)
#define pr5(a,b,c,d,e) pr1(a)<<pr4(b,c,d,e)
#define pr6(a,b,c,d,e,f) pr1(a)<<pr5(b,c,d,e,f)
#define pr7(a,b,c,d,e,f,g) pr1(a)<<pr6(b,c,d,e,f,g)
#define pr8(a,b,c,d,e,f,g,h) pr1(a)<<pr7(b,c,d,e,f,g,h)
#define prArr(a) {cerr<<(#a)<<"={";int i=0;for(auto t:(a))cerr<<(i++?", ":"")<<t;cerr<<"}"<<endl;}
using namespace std;
using Int = long long;
using _int = int;
using ll = long long;
using Double = long double;
const Int INF = (1LL<<60)+1e9; // ~ 1.15 * 1e18
const Int mod = (1e9)+7;
const Double EPS = 1e-8;
const Double PI = 6.0 * asin((Double)0.5);
using P = pair<Int,Int>;
template<class T> T Max(T &a,T b){return a=max(a,b);}
template<class T> T Min(T &a,T b){return a=min(a,b);}
template<class T1, class T2> ostream& operator<<(ostream& o,pair<T1,T2> p){return o<<"("<<p.first<<","<<p.second<<")";}
template<class T1, class T2, class T3> ostream& operator<<(ostream& o,tuple<T1,T2,T3> t){
  return o<<"("<<get<0>(t)<<","<<get<1>(t)<<","<<get<2>(t)<<")";}
template<class T1, class T2> istream& operator>>(istream& i,pair<T1,T2> &p){return i>>p.first>>p.second;}
template<class T> ostream& operator<<(ostream& o,vector<T> a){Int i=0;for(T t:a)o<<(i++?" ":"")<<t;return o;}
template<class T> istream& operator>>(istream& i,vector<T> &a){for(T &t:a)i>>t;return i;}


signed main(){
  srand((unsigned)time(NULL));
  cin.tie(0);
  ios_base::sync_with_stdio(0);
  cout << fixed << setprecision(12);
  while(1){
    Int n, m;
    cin>>n>>m;
    if(n == 0 && m == 0) break;
    vector<Int> S(n);
    cin>>S;

    auto simulate = [&](Int X){
      Int level = 1;    
      auto getNextMonster = [&](){
	Int i = lower_bound(S.begin(), S.end(), level) - S.begin();
	Int j = i - 1;
	Int res = -1;
	if(0 <= i && i < (Int)S.size() && level + X > S[i]) res = i;
	if(0 <= j && j < (Int)S.size() && level + X > S[j] &&
	   (res == -1 || abs(level - S[i]) > abs(level - S[j]))) res = j;
	return res;
      };
      
      Int cnt = 0;
      while(1){
	cnt++;
	Int nextMonster = getNextMonster();
	if(level + X > S[n-1]) return cnt;
	if(nextMonster == -1) return INF;
	level += max(1LL, X - abs(level - S[nextMonster]) );
      }
      return cnt;
    };


    Int L = 1, R = 1e6 + 100;
    while(L + 1 < R){
      Int M = (L + R) / 2;
      Int cnt = simulate(M);
      (cnt < m)? (R = M):(L = M);
    }
    
    Int ans = L;
    Int cnt = simulate(ans);
    if(cnt < m || cnt == INF) ans = -1;
    cout<<ans<<endl;
  }
  return 0;
}

