import java.io.*;
import java.util.StringTokenizer;
public class Main {
public static void main(String[] args) {
Kattio kattio =new Kattio();
int N = kattio.nextInt();
int[] plants = new int[N];
for (int i = 0; i < N; i++) {
plants[i] = kattio.nextInt();
}
long[] left = new long[N];//left[i] = min increase to fix the pair i, i+1
long[] right = new long[N]; //total increases needed on prefix to make array strictly decreasing after i
for (int i = 0; i < N-1; i++) {
if(plants[i] > plants[i+1]){
left[i] = 0;
}
else{
left[i] = plants[i+1]-plants[i]+1;
}
}
left[N-1]=0;
for (int i = N-1; i >0 ; i--) {
if(plants[i] > plants[i-1]){
right[i] =0;
}else{
right[i] = plants[i-1]-plants[i]+1;
}
}
right[0] = 0;
for (int i = 1; i <N ; i++) {
right[i] +=right[i-1];
}
for (int i = N-2; i >=0 ; i--) {
left[i] += left[i+1];
}
long min = Long.MAX_VALUE;
for (int i = 0; i < N; i++) { //iterate over all "peak" points
min = Math.min(min, Math.max(left[i], right[i]));
}
kattio.println(min);
kattio.close();
}
static class Kattio extends PrintWriter {
private BufferedReader r;
private StringTokenizer st;
public Kattio() {
this(System.in, System.out);
}
public Kattio(InputStream i, OutputStream o) {
super(o);
r = new BufferedReader(new InputStreamReader(i));
}
public String next() {
try {
while (st == null || !st.hasMoreTokens())
st = new StringTokenizer(r.readLine());
return st.nextToken();
} catch (Exception e) {
return null;
}
}
public int nextInt() {
return Integer.parseInt(next());
}
public long nextLong() {
return Long.parseLong(next());
}
@Override
public void close() {
super.close();
try {
r.close();
} catch (IOException ignored) {
}
}
}
}
