#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
#define pb push_back
#define pii pair<int, int>
#define pll pair<ll, ll>
#define st first
#define nd second
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define debug false
const int MAXN = 200 * 1000;
ll a[MAXN];
ll r[MAXN];
ll m[MAXN];
int main () {
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n;
cin >> n;
for (int i = 0; i < n; i ++) {
cin >> a[i];
}
for (int i = 1; i < n; i ++) {
r[i] = r[i - 1] + max(0LL, a[i - 1] - a[i] + 1LL);
}
for (int i = n - 2; i >= 0; i --) {
m[i] = m[i + 1] + max(0LL, a[i + 1] - a[i] + 1LL);
}
/*for (int i = 0; i < n; i ++) {
cout << r[i] << " " << m[i] << "\n";
}*/
ll wyn = LLONG_MAX;
for (int i = 0; i < n; i ++) {
wyn = min(wyn, max(r[i], m[i]));
}
cout << wyn << "\n";
return 0;
}
