#include <bits/stdc++.h>
using namespace std;
#define loop(i,b,e) for(int i=b;i<=e;i++)
#define loop2(i,e,b) for(int i=e;i>=b;i--)
#define io ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
#define fi first
#define se second
typedef long long ll;
typedef unsigned long long ull;
const int N = 1e6 + 5;
const ll mod = 1e9 + 7;
const int oo = 1e9;

ll t[N];
int n;
int dp[N][3];

int solve(int i,int last,ll prog) {
    if (i == n) return 0;
    ll curr;
    if (last == 0) curr = t[i-1];
    else if (last == 1) curr = t[i-1] + 1;
    else curr = t[i-1] - 1;
    int ret = oo;
    if (prog == -1) {
        ret = min(ret,solve(i + 1, 0, t[i] - curr));
        ret = min(ret, 1 + solve(i + 1, 1, t[i] - curr+1));
        ret = min(ret, 1 + solve(i + 1, 2, t[i] - curr-1));
    }
    else {
        if(t[i]-curr==prog) ret = min(ret, solve(i + 1, 0, t[i] - curr));
        if(t[i]-curr+1==prog) ret = min(ret, 1 + solve(i + 1, 1, t[i] - curr + 1));
        if(t[i]-curr-1==prog) ret = min(ret, 1 + solve(i + 1, 2, t[i] - curr - 1));
    }
    return ret;
}

int main() {
    scanf("%d",&n);
    memset(dp,-1,sizeof dp);
    loop(i, 0, n - 1)
        scanf("%lld",&t[i]);
    int ans = min(solve(1, 0, -1), min(1 + solve(1, 1, -1), 1 + solve(1, 2, -1)));
    cout << (ans==oo ? -1 : ans);
}