// created : 2026/02/04 19:05:51
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define all(v) v.begin(), v.end()
void solve() {
int n;
cin >> n;
int arr[n + 1], pref[n + 1], suff[n + 2];
for(int i = 1; i <= n; i++) {
cin >> arr[i];
}
pref[1] = 0;
for(int i = 2; i <= n; i++) {
pref[i] = pref[i - 1];
if(arr[i] < arr[i - 1]) {
pref[i] += (arr[i - 1] - arr[i] + 1);
}
}
suff[n] = 0, suff[n + 1];
for(int i = n - 1; i >= 1; i--) {
suff[i] = suff[i + 1];
if(arr[i] < arr[i + 1]) {
suff[i] += (arr[i + 1] - arr[i] + 1);
}
}
int ans = LLONG_MAX;
for(int i = 2; i <= n - 1; i++) {
ans = min(ans, max(pref[i], suff[i]));
}
cout << ans << '\n';
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int t = 1;
//cin >> t;
while (t--) {
solve();
}
}
