#include <bits/stdc++.h>
#define For(i, a, b) for(int (i)=(int)(a); (i)<(int)(b); ++(i))
#define rFor(i, a, b) for(int (i)=(int)(a)-1; (i)>=(int)(b); --(i))
#define rep(i, n) For(i, 0, n)
#define rrep(i, n) rFor(i, n, 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;
template<class T> bool chmax(T &a, const T &b){if(a<b){a=b; return true;} return false;}
template<class T> bool chmin(T &a, const T &b){if(a>b){a=b; return true;} return false;}
template<class T> T div_floor(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>=0 ? a/b : (a+1)/b-1;
}
template<class T> T div_ceil(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>0 ? (a-1)/b+1 : a/b;
}

constexpr lint mod = 1000000007;
constexpr lint INF = mod * mod;
constexpr int MAX = 100010;

int n, m, s[MAX];

int check(int mid){
    int cur = 1, cnt = 0;
    while(s[n - 1] >= cur + mid){
        int lv = -1;
        auto itr = lower_bound(s, s + n, cur);
        if(itr - s > 0) chmax(lv, max(1, mid - abs(cur - *prev(itr))));
        if(*itr < cur + mid) chmax(lv, max(1, mid - abs(cur - *itr)));
        if(lv < 0) return mod;
        cur += lv;
        ++cnt;
    }
    return cnt + 1;
}

void solve(){
    rep(i, n) scanf("%d", &s[i]);
    int low = 1, high = s[n - 1];
    if(check(1) < m){
        printf("%d\n", -1);
        return;
    }
    while(high - low > 1){
        int mid = (high + low) / 2;
        (check(mid) >= m ? low : high) = mid;
    }
    printf("%d\n", (check(low) < mod ? low : -1));
}

int main(){
    while(scanf("%d%d", &n, &m) && n){
        solve();
    }
}
