#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define FOR(i, l, r, d) for(int i=(l); i<=(r); i+=(d))
#define szof(x) ((int)(x).size())
#define vi vector<int>
#define pii pair<int, int>
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define mkp make_pair
const int INF = INT_MAX;
const int LNF = INF*INF;
const int MOD = 1000000007;
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
const int MAX = 200010;
int n;
int arr[MAX];
int dif[MAX];
int cost_i[MAX], cost_d[MAX];
int pref[MAX], suf[MAX];
int res;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n;
FOR(i, 1, n, 1){
cin>>arr[i];
dif[i] = arr[i] - arr[i-1];
cost_i[i] = max((int)0, 1 - dif[i]);
cost_d[i] = max((int)0, 1 + dif[i]);
}
FOR(i, 1, n, 1){
pref[i] = pref[i-1] + cost_i[i];
}
for(int i=n; i>=1; i--){
suf[i] = suf[i+1] + cost_d[i];
}
res = min(pref[n], suf[1]);
FOR(i, 1, n-1, 1){
res = min(res, max(pref[i], suf[i+1]));
}
cout<<res<<'\n';
return 0;
}
