#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 = 1e15 + 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++)
{
// cout << abs(v[i] - v[i+1]) << " ";
if(v[i] >= v[i+1])
s1[i+1] += abs(v[i] - v[i+1]) + 1;
s1[i+1] += s1[i];
}
for(int i = n-1; i > 0; i--)
{
if(v[i] >= v[i-1])
s2[i-1] += abs(v[i] - v[i-1]) + 1;
s2[i-1] += s2[i];
}
int ans = Inf;
for(int i = 0; i < n; i++)
{
int k = s1[i] + v[i], t = 0;
if(i != n - 1)
{
if(v[i+1] >= v[i]){
t -= abs(v[i] - v[i+1]) + 1;
if(k <= v[i+1])
t += abs(k - v[i+1]) + 1;
}
}
ans = min(ans, s1[i] + s2[i] + t);
}
/*for(int i = 0; i < n; i++) cerr << s1[i] << " ";
cerr << endl;
for(int i = 0; i < n; i++) cerr << s2[i] << " ";
cerr << endl;*/
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;
}
