#include<bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 1e6 + 66;
ll A[N], b[N];
int main() { // fuck O(N) all my homies brute for O(N^2)
ios_base::sync_with_stdio(0);
cin.tie(0), cout.tie(0);
int n;
cin >> n;
for (int i = 1 ; i <= n ; ++ i) {
cin >> A[i];
}
ll ans = ll(1e18);
for (int i = 1 ; i <= n ; ++ i) { // the highest point
for (int j = 1 ; j <= n ; ++ j) {
b[j] = A[j];
}
ll L = 0, R = 0; // the number of waterings of the left side and the right side
for (int j = 1 ; j + 1 < i ; ++ j) {
if (b[j + 1] < b[j] + 1) {
L += b[j] + 1 - b[j + 1];
b[j + 1] = b[j] + 1;
}
}
for (int j = n ; j - 1 > i ; -- j) {
if (b[j - 1] < b[j] + 1) {
R += b[j] + 1 - b[j - 1];
b[j - 1] = b[j] + 1;
}
}
ll nL = 0, nR = 0;
if (b[i - 1] + 1 > b[i] + R) {
nL = b[i - 1] + 1 - b[i] - R;
}
if (b[i + 1] + 1 > b[i] + L) {
nR = b[i + 1] + 1 - b[i] - L;
}
L += nL, R += nR;
ans = min(ans, max(L, R));
}
cout << ans;
}
