#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
int n; cin >> n;
vector<int> a(n), pref(n), suf(n);
for (int i = 0; i < n; i++){
cin >> a[i];
}
for (int i = 1; i < n; i++){
pref[i] = pref[i - 1];
if (a[i] <= a[i - 1]){
pref[i] += (a[i - 1] - a[i] + 1);
}
}
for (int i = n - 2; i >= 0; i--){
suf[i] = suf[i + 1];
if (a[i] <= a[i + 1]){
suf[i] += (a[i + 1] - a[i] + 1);
}
}
int ans = 1e18 + 7;
for (int i = 0; i < n; i++){
ans = min(ans, max(suf[i], pref[i]));
}
cout << ans << endl;
}
