#include <bits/stdc++.h>
#define ll long long
#define ull unsigned ll
#define pb push_back
#define pa pair<int, int>
#define pall pair<ll, int>
#define fi first
#define se second
#define TASK "test"
#define Size(x) (int) x.size()
#define all(x) x.begin(), x.end()
using namespace std;
template<typename T1, typename T2> bool mini (T1 &a, T2 b) {if(a > b) a = b; else return 0; return 1;}
template<typename T1, typename T2> bool maxi (T1 &a, T2 b) {if(a < b) a = b; else return 0; return 1;}
const int MOD = 1e9 + 7;
const int LOG = 20;
const int maxn = 2e5 + 7;
const ll oo = 1e18 + 69;
int n, a[maxn], pre_d[maxn], suf_d[maxn], val_pre[maxn], val_suf[maxn];
ll pre[maxn], suf[maxn];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//freopen(TASK".inp", "r", stdin);
//freopen(TASK".out", "w", stdout);
cin>>n;
for(int i = 1; i <= n; i++) cin>>a[i];
val_pre[1] = a[1];
for(int i = 2; i <= n; i++) {
val_pre[i] = max(val_pre[i - 1] + 1, a[i]);
pre_d[i] = val_pre[i] - a[i];
pre[i] = pre[i - 1] + max(0, pre_d[i] - pre_d[i - 1]);
}
val_suf[n] = a[n];
for(int i = n - 1; i >= 1; i--) {
val_suf[i] = max(val_suf[i + 1] + 1, a[i]);
suf_d[i] = val_suf[i] - a[i];
suf[i] = suf[i + 1] + max(0, suf_d[i + 1] - suf_d[i]);
}
ll ans = oo;
for(int i = 1; i <= n; i++) {
int val = max(val_pre[i], val_suf[i]);
int tmp = val - a[i];
ll l = pre[i - 1] + max(0, tmp - pre_d[i - 1]);
ll r = suf[i + 1] + max(0, suf_d[i + 1] - tmp);
mini(ans, max(l, r));
}
cout<<ans;
return 0;
}
