#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define db double
#define ll __int128
#define int long long
#define pb push_back
#define fs first
#define sd second
#define Mod long(1e9 + 7)
#define all(x) x.begin(), x.end()
#define unvisited long(-1)
#define Eps double(1e-9)
#define _for(i, n) for(int i = 0; i < (n); i++)
#define dbg(x) cout << #x ": " << x << endl;
const int Max = 1e6 + 7, Inf = 1e9 + 7;
void print(bool x) { cout << (x ? "YES" : "NO") << endl; }
string tostring (__int128 x)
{
string ans = "";
while(x > 0)
{
ans += (x % 10 + '0');
x /= 10;
}
reverse(all(ans));
return ans;
}
void solve()
{
int n; cin >> n;
vector <int> v(n), s1(n), s2(n);
for(auto& u : v) cin >> u;
for(int i = 0; i < n-1; i++)
s1[i+1] = max(0LL, (v[i] + 1) - v[i+1]) + s1[i];
for(int i = n-1; i > 0; i--)
s2[i-1] = max(0LL, (v[i] + 1) - v[i-1]) + s2[i];
int ans = Inf;
for(int i = 0; i < n; i++)
ans = min(ans, max(s1[i], s2[i]));
cout << ans << endl;
}
int32_t main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int Q = 1; //cin >> Q;
while (Q--)
{
solve();
}
return 0;
}
