#include<bits/stdc++.h>
typedef long long ll;
#define pii pair<ll, ll>
#define fi first
#define se second
#define endl '\n'
#define TASK ""
#define N 800005
#define LOG 17
using namespace std;
ll n, a[N], pf[N], sf[N];
signed main() {
// freopen("test.inp", "r", stdin);
//freopen("test.out", "w", stdout);
//srand(time(0));
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n;
for (int i = 1; i <= n; i ++) {
cin >> a[i];
}
for (int i = 2; i <= n; i ++) {
pf[i] = pf[i - 1] + max(0LL, a[i - 1] + 1 - a[i]);
}
for (int i = n - 1; i >= 1; i --) {
sf[i] = sf[i + 1] + max(0LL, a[i + 1] - a[i] + 1);
}
ll ans = 1e18;
for (int i = 1; i <= n; i ++) {
ans = min(ans, max(pf[i], sf[i]));
}
cout << ans;
}
