#include<bits/stdc++.h>
#define fi first
#define se second
#define int long long
#define sz(v) (int)v.size()
using namespace std;
const int MAXN = 2e5 + 5;
int n,a[MAXN], prefix[MAXN], suffix[MAXN];
signed main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> n;
for (int i = 1;i <= n;i++) cin >> a[i];
vector<int> diff;
for (int i = 1;i < n;i++) {
diff.push_back(a[i] - a[i + 1]);
}
if (diff[0] >= 0) prefix[0] = diff[0] + 1;
else prefix[0] = 0;
for (int i = 1;i < sz(diff);i++) {
if (diff[i] >= 0) prefix[i] = prefix[i - 1] + diff[i] + 1;
else prefix[i] = prefix[i - 1];
}
if (diff[sz(diff) - 1] <= 0) suffix[sz(diff) - 1] = abs(diff[sz(diff) - 1]) + 1;
else suffix[sz(diff) - 1] = 0;
for (int i = sz(diff) - 2;i >= 0;i--) {
if (diff[i] <= 0) suffix[i] = suffix[i + 1] + abs(diff[i]) + 1;
else suffix[i] = suffix[i + 1];
}
// ---- ++++
int ans = 1e18;
for (int i = 0;i < sz(diff);i++) {
int p = (i == 0) ? 0 : prefix[i - 1];
int s = suffix[i];
ans = min(ans, max(p,s));
}
cout << ans;
return 0;
}
