#include<bits/stdc++.h>
using namespace std;
int main()
{
long long n;
cin>>n;
vector<long long>a(n+1);
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
if (n == 1) {
cout << 0 << endl;
return 0;
}
vector<long long>d(n);
for (int i = 1; i < n; i++) {
d[i] = a[i+1] - a[i];
}
vector<long long> p(n + 1, 0);
for (int i = 1; i < n; i++) {
p[i+1] = p[i] + max(0LL, 1 - d[i]);
}
vector<long long>s(n + 1, 0);
for (int i = n - 1; i >= 1; i--) {
s[i] = s[i+1] + max(0LL, d[i] + 1);
}
long long minn = -1;
for (int k = 1; k <= n; k++) {
long long curr = max(p[k], s[k]);
if (minn == -1 || curr < minn) {
minn = curr;
}
}
cout << minn << endl;
}
