#include <bits/stdc++.h>
#define pb emplace_back
#define SZ(x) ((int)x.size())
#define fi first
#define se second
#define FOR(i, a, b) for (int i = a, _b = b; i <= _b; ++i)
#define FORD(i, a, b) for (int i = a, _b = b; i >= _b; --i)
#define FORLL(i, a, b) for (ll i = a, _b = b; i <= _b; ++i)
#define FORDLL(i, a, b) for (ll i = a, _b = b; i >= _b; --i)
#define all(x) x.begin(), x.end()
#define uni(x) sort(all(x)), x.erase(unique(all(x)), x.end())
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define dbg(...) debug(#__VA_ARGS__, __VA_ARGS__)
template<typename T>
void __prine_one(const char *&s, const T &x)
{
while (*s == ' ') ++s;
const char *p = s;
int bal = 0;
while (*s)
{
if (*s == '(') ++bal;
else if (*s == ')') --bal;
else if (*s == ',' && bal == 0) break;
++s;
}
cerr.write(p, s - p) << " = " << x;
if (*s == ',')
{
cerr << " , ";
++s;
}
}
template<typename... Args>
void debug(const char *s, Args... args)
{
cerr << "[ ";
int dummy[] = {0, (__prine_one(s, args), 0)...};
(void)dummy;
cerr << " ]\n\n";
}
template<class X>
bool maximize(X &a, const X &b)
{
if (b > a)
{
a = b;
return true;
}
return false;
}
template<class X>
bool minimize(X &a, const X &b)
{
if (b < a)
{
a = b;
return true;
}
return false;
}
// --------------------------------------------------------------------------------------------
constexpr int maxn = 2e5 + 1;
int n, a[maxn]; ll low, high;;
// --------------------------------------------------------------------------------------------
void readInput()
{
cin >> n;
FOR(i, 1, n)
cin >> a[i];
FORD(i, n, 1)
{
a[i] = a[i] - a[i - 1];
if (i > 1)
high += max(0, a[i] + 1);
}
}
void solve()
{
ll res = 1e18;
FOR(i, 1, n)
{
res = min(res, max(low, high));
high -= max(0, a[i + 1] + 1);
low += max(0, -a[i + 1] + 1);
}
cout << res;
}
signed main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define TASK "TEST"
if (fopen(TASK".INP", "r"))
{
freopen(TASK".INP", "r", stdin);
freopen(TASK".OUT", "w", stdout);
}
readInput();
solve();
return 0;
}
