#include <bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
//using namespace __gnu_pbds;
#define int long long
#define mod 1000000007
#define base 7001
#define base2 757
// #define pi acos(-1)
#define double long double
//#define ordered_set tree<int, null_type, less<int>, rb_tree_tag,tree_order_statistics_node_update>
//#define ordered_multiset tree<int, null_type, less_equal<int>, rb_tree_tag,tree_order_statistics_node_update>
#pragma GCC optimize("O3,Ofast,unroll-loops")
#pragma GCC target("avx2,sse3,sse4,avx")
constexpr int maxn = 1000001;
const int N = 1 << (int)(ceil(log2(maxn)));
int n, a[maxn], b[maxn];
int prf[maxn][2], suf[maxn][2];
signed main()
{
cin.tie(0) -> sync_with_stdio(0);
cin >> n;
for (int i = 1; i <= n; i++)
cin >> a[i];
for (int i = n; i >= 1; i--)
{
b[i] = a[i];
suf[i][0] = suf[i + 1][0];
suf[i][0] += max((a[i + 1] + 1 - a[i]) - suf[i + 1][0], 0ll);
suf[i][1] = a[i] + max(a[i + 1] + 1 - a[i], 0ll);
a[i] += max(a[i + 1] + 1 - a[i], 0ll);
}
for (int i = 1; i <= n; i++)
{
a[i] = b[i];
prf[i][0] = prf[i - 1][0];
prf[i][0] += max((a[i + 1] + 1 - a[i]) - prf[i - 1][0], 0ll);
prf[i][1] = a[i] + max(a[i - 1] + 1 - a[i], 0ll);
a[i] += max(a[i - 1] + 1 - a[i], 0ll);
}
int mn = INT_MAX;
for (int i = 1; i <= n; i++)
{
int idx = i, jdx = i;
if (suf[i][1] > prf[i][1])
idx--;
else
jdx++;
mn = min(mn, suf[jdx][0] + prf[idx][0]);
}
cout << mn;
}
