#include<bits/stdc++.h>
#define fi first
#define se second
#define int long long
using namespace std;
using ll = long long;
using ii = pair<int, int>;
using aa = array<int,3>;
const int N = 5e5+5;
int n, ans = (int)1e18, timer, cur, pos, mx;
int a[N], pre[N], suf[N];
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);cout.tie(NULL);
cin >> n;
for (int i = 1; i <= n; i++){
cin >> a[i];
}
for (int i = 2; i <= n; i++){
pre[i] = pre[i-1];
if (a[i-1] >= a[i]) pre[i] += (a[i-1] + 1 - a[i]);
}
for (int i = n - 1; i >= 1; i--){
suf[i] = suf[i + 1];
if (a[i + 1] >= a[i]) suf[i] += (a[i + 1] + 1 - a[i]);
}
for (int i = 1; i <= n; i++) ans = min(ans,max(pre[i],suf[i]));
cout << ans;
return 0;
}
