#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int n;
ll a[200005], vall[200005], valr[200005];
ll difl[200005], difr[200005];
ll qsl[200005], qsr[200005];
ll ans = LLONG_MAX;
int main() {
cin.tie(0) -> sync_with_stdio(0);
cin >> n;
for(int i=1;i<=n;i++) cin >> a[i];
for(int i=1;i<=n;i++) vall[i] = max(vall[i-1] + 1, a[i]);
for(int i=n;i>=1;i--) valr[i] = max(valr[i+1] + 1, a[i]);
for(int i=1;i<=n;i++) {
difl[i] = vall[i] - a[i];
qsl[i] = qsl[i-1] + difl[i];
}
for(int i=n;i>=1;i--) {
difr[i] = valr[i] - a[i];
qsr[i] = qsr[i+1] + difr[i];
}
ans = min(qsl[n], qsr[1]);
for(int i=2;i<=n;i++) {
ans = min(ans, max(qsl[i-1], qsr[i]) + (vall[i-1] == valr[i]));
}
cout << ans;
}
