#include "bits/stdc++.h"
using namespace std;
#define pb push_back
#define pf push_front
#define ppb pop_back
#define ppf pop_front
#define all(v) v.begin(),v.end()
#define FILE(v) freopen(v".inp", "r", stdin),freopen(v".out", "w", stdout)
#define int ll
typedef long long ll;
typedef long double ld;
///------------------------------------------------///
/// VARIABLE
const int INF = 1e18;
const int MAXN = 2e5 + 5;
int n;
int a[MAXN];
int L[MAXN], R[MAXN];
int reqL[MAXN], reqR[MAXN];
int cL[MAXN], cR[MAXN];
/// FUNCTIONS
/// PROCESS
void read()
{
cin >> n;
for (int i = 1; i <= n; i++) cin >> a[i];
}
void solve()
{
for (int i = 1; i <= n; i++)
{
L[i] = max(L[i - 1] + 1, a[i]);
reqL[i] = L[i] - a[i];
cL[i] = cL[i - 1] + max(0LL, reqL[i] - reqL[i - 1]);
}
for (int i = n; i >= 1; i--)
{
R[i] = max(R[i + 1] + 1, a[i]);
reqR[i] = R[i] - a[i];
cR[i] = cR[i + 1] + max(0LL, reqR[i] - reqR[i + 1]);
}
// for (int i = 1; i <= n; i++) cout << cL[i] << ' ';
// cout << '\n';
// for (int i = 1; i <= n; i++) cout << cR[i] << ' ';
// cout << '\n';
int ans = INF;
for (int i = 1; i <= n; i++)
{
ans = min(ans, cL[i] + cR[i] - min(reqL[i], reqR[i]));
}
cout << ans << '\n';
}
/// MAIN
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0), cout.tie(0);
// FILE("test");
int T = 1;
bool multitest = false;
if (multitest) cin >> T;
double st = clock();
while (T--)
{
read();
solve();
}
double en = clock();
cerr << fixed << setprecision(3) << (en - st) / CLOCKS_PER_SEC;
return 0;
}
