#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(), x.end()
#define ar array
#define pb push_back
#define ln '\n'
#define int long long
using i64 = long long;
template <class F, class _S>
bool chmin(F &u, const _S &v){
bool flag = false;
if ( u > v ){
u = v; flag |= true;
}
return flag;
}
template <class F, class _S>
bool chmax(F &u, const _S &v){
bool flag = false;
if ( u < v ){
u = v; flag |= true;
}
return flag;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n; cin >> n;
vector <int> d(n - 1);
int lst = -1;
for ( int i = 0; i < n; i++ ){
int x; cin >> x;
if ( lst != -1 ){
d[i - 1] = x - lst;
} lst = x;
}
vector <int> pf(n), sf(n);
for ( int i = 0; i + 1 < n; i++ ){
pf[i + 1] = pf[i] + min(0LL, d[i] - 1);
}
for ( int i = n - 2; i >= 0; i-- ){
sf[i] = sf[i + 1] + max(0LL, d[i] + 1);
}
int opt = 1e18;
for ( int i = 0; i < n; i++ ){
chmin(opt, max(sf[i], -pf[i]));
}
cout << opt;
cout << '\n';
}
