#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int N = 2e5+10;
ll a[N], s[N], p[N], cost_pref[N], cost_suff[N];
ll n;
int main() {
ios::sync_with_stdio(0); cin.tie(0);
cin >> n;
for(int i = 1; i <= n; ++i) {
cin >> a[i];
p[i] = s[i] = a[i];
}
ll cnt = 0, cost = 0;
cost_pref[1] = 0;
for(int i = 2; i <= n; ++i) {
p[i] += cnt;
if(p[i] <= p[i - 1]) {
cost += p[i - 1] + 1 - p[i];
}
cost_pref[i] = cost;
}
cnt = cost = 0;
cost_suff[n] = 0;
for(int i = n - 1; i >= 1; --i) {
s[i] += cnt;
if(s[i] <= s[i + 1]) {
cost += s[i + 1] + 1 - s[i];
}
cost_suff[i] = cost;
}
ll ans = min(cost_suff[1], cost_pref[n]);
for(int i = 1; i < n; ++i) {
//i | i + 1
if(cost_pref[i] == cost_suff[i+1]) {
ans = min(ans, cost_pref[i] + (p[i] == s[i+1]));
} else {
ans = min(ans, max(cost_pref[i], cost_suff[i+1]));
}
}
cout << ans << endl;
}
