#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define endl '\n'
#define MASK(i) (1LL << (i))
#define ull unsigned long long
#define ld long double
#define pb push_back
#define all(x) (x).begin() , (x).end()
#define BIT(x , i) ((x >> (i)) & 1)
#define TASK "task"
#define sz(s) (int) (s).size()
using namespace std;
const int mxN = 5e5 + 227;
const int inf = 1e9 + 277;
const int mod = 1e9 + 7;
const ll infll = 1e18 + 7;
const int base = 307;
const int LOG = 20;
template <typename T1, typename T2> bool minimize(T1 &a, T2 b) {
if (a > b) {a = b; return true;} return false;
}
template <typename T1, typename T2> bool maximize(T1 &a, T2 b) {
if (a < b) {a = b; return true;} return false;
}
int n;
ll a[mxN];
ll b[mxN];
int lef[mxN];
int rig[mxN];
ll pre[mxN];
ll suf[mxN];
void solve()
{
cin >> n;
for(int i = 1; i <= n; i++) cin >> a[i];
b[1] = a[1];
for(int i = 2; i <= n; i++) {
b[i] = a[i] + pre[i - 1];
pre[i] = pre[i - 1] + max(0LL, b[i - 1] + 1 - b[i]);
b[i] = max(b[i], b[i - 1] + 1);
}
b[n] = a[n];
for(int i = n - 1; i >= 1; i--) {
b[i] = a[i] + suf[i + 1];
suf[i] = suf[i + 1] + max(0LL, b[i + 1] + 1 - b[i]);
b[i] = max(b[i], b[i + 1] + 1);
}
ll res = infll;
for(int i = 1; i <= n; i++)
minimize(res, max(pre[i], suf[i]));
cout << res;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
//freopen(TASK".inp" , "r" , stdin);
//freopen(TASK".out" , "w" , stdout);
int tc = 1;
//cin >> tc;
while(tc--) {
solve();
}
return 0;
}
