#include <bits/stdc++.h>
#define all(x) (x).begin(), (x).end()
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pull;
typedef pair<int, int> pii;
typedef pair<ld, ld> pld;
ll n, a[200009];
void calc(ll cur[]){
cur[0] = 0;
for(ll i = 1; i < n; ++i){
cur[i] = cur[i-1];
cur[i] += max(0LL, a[i-1]+1-a[i]);
}
}
ll cur[2][200009];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//freopen("in.txt", "r", stdin);
//freopen("out.txt", "w", stdout);
cin >> n;
for(ll i = 0; i < n; ++i)
cin >> a[i];
calc(cur[0]);
reverse(a, a+n);
calc(cur[1]);
reverse(cur[1], cur[1]+n);
ll res = 1e18;
for(ll i = 0; i < n; ++i){
res = min(res, max(cur[0][i], cur[1][i]));
//cout << i << ' ' << max(cur[0][i], cur[1][i]) << '\n';
}
cout << res << '\n';
}
