#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5 + 5;
const long long INF = 1e18;
int n;
long long A[MAXN];
long long B[MAXN];
long long Pos[MAXN], Neg[MAXN];
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> n;
for(int i = 1; i <= n; i++)
{
cin >> A[i];
if(i != 1)
{
B[i - 1] = A[i - 1] - A[i];
}
}
for(int i = 1; i <= n; i++)
{
Pos[i] = Pos[i - 1];
if(B[i] >= 0)
{
Pos[i] += (B[i] + 1);
}
}
for(int i = n - 1; i >= 1; i--)
{
Neg[i] = Neg[i + 1];
if(B[i] <= 0)
{
Neg[i] += (1 - B[i]);
}
}
long long ans = INF;
for(int i = 1; i <= n; i++)
{
ans = min(ans, max(Pos[i - 1], Neg[i]));
}
cout << ans << '\n';
}
