#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], suf[maxn];
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 > 0; i--)
{
suf[i] = suf[i + 1];
suf[i] += max(a[i + 1] - a[i] + 1, 0ll);
}
for (int i = 1; i <= n; i++)
{
prf[i] = prf[i - 1];
prf[i] += max(a[i - 1] - a[i] + 1, 0ll);
}
int mn = LLONG_MAX;
for (int i = 1; i <= n; i++)
{
int idx = i, jdx = i;
mn = min(mn, max(suf[jdx], prf[idx]));
}
cout << mn;
}
