#include<iostream>
#include<stdio.h>
#include<stdint.h>
#include<vector>
#include<algorithm>
#include<utility>
#include<set>
#include<map>
#include<queue>
#include<stack>
#include<deque>
#include<string>
#include<assert.h>
using namespace std;
const int64_t INF = (int64_t) 1e18 + 5;
const int64_t mINF = 1e6 + 1;
const int64_t MOD = (int64_t) 1e9 + 7;
const int nbit = 30;
const int ndig = 10;
const int nchar = 26;
const int p1 = 31;
const int p2 = 53;
const int D = 4;
int dr[D] = {0, 1, 0, -1};
int dc[D] = {1, 0, -1, 0};
// 0 right // 1 down // 2 left // 3 up
struct Solution
{
int n;
vector<int> a;
Solution() {}
void solve()
{
cin >> n;
a.resize(n);
for(int i = 0; i < n; i++)
{
cin >> a[i];
}
vector<int64_t> prefix(n, 0);
vector<int64_t> suffix(n, 0);
int64_t max_ = a[0];
for(int i = 0; i < n; i++)
{
int64_t tmp = 0;
if(i != 0) tmp = prefix[i - 1];
int64_t need = max_ - (a[i] + tmp);
prefix[i] += tmp;
if(need > 0) prefix[i] += need;
max_ = max(max_, a[i] + tmp) + 1;
}
max_ = a[n - 1];
for(int i = n - 1; i >= 0; i--)
{
int64_t tmp = 0;
if(i != n - 1) tmp = suffix[i + 1];
int64_t need = max_ - (a[i] + tmp);
suffix[i] = tmp;
if(need > 0) suffix[i] += need;
max_ = max(max_, a[i] + tmp) + 1;
}
int64_t ans = INF;
for(int i = 0; i < n; i++)
{
ans = min(ans, max(prefix[i], suffix[i]));
}
cout << ans << "\n";
}
int modsub(int t1, int t2)
{
int res = t1 - t2;
if(res < 0) res += MOD;
return res;
}
int modadd(int t1, int t2)
{
int res = t1 + t2;
if(res >= MOD) res -= MOD;
return res;
}
int modmul(int t1, int t2)
{
int64_t res = 1LL * t1 * t2;
return (res % MOD);
}
bool BIT(int64_t& mask, int& j)
{
return (mask & MASK(j));
}
int64_t MASK(int j)
{
return (1LL << j);
}
};
void __startup()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
// freopen("in.txt", "r", stdin);
// freopen("out.txt", "w", stdout);
}
int main()
{
__startup();
int t = 1;
// cin >> t;
for(int i = 1; i <= t; i++)
{
Solution().solve();
}
return 0;
}
