#include <bits/stdc++.h>
using namespace std;
#define lli long long int
#define pb push_back
#define eb emplace_back
#define pii pair<int,int>
#define X first
#define Y second
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
void abc() {cout << endl;}
template <typename T, typename ...U> void abc(T i, U ...j) {
cout << i << ' ', abc(j...);
}
template <typename T> void printv(T l, T r) {
for (; l != r; ++l) cout << *l << " \n"[l + 1 == r];
}
#ifdef Doludu
#define test(x...) abc("[" + string(#x) + "]", x)
#define owo freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout)
#else
#define test(x...) void(0)
#define owo ios::sync_with_stdio(false), cin.tie(0)
#endif
const int N = 200000;
int main () {
owo;
int n;
cin >> n;
vector <int> a(n);
for (int i = 0; i < n; ++i)
cin >> a[i];
vector <int> d(n - 1);
for (int i = 0; i < n - 1; ++i)
d[i] = a[i + 1] - a[i];
vector <lli> pre(n), suf(n);
for (int i = 1; i < n; ++i) {
pre[i] = pre[i - 1] + (d[i - 1] <= 0 ? -d[i - 1] + 1 : 0);
}
for (int i = n - 2; ~i; --i) {
suf[i] = suf[i + 1] + (d[i] >= 0 ? d[i] + 1 : 0);
}
lli ans = 1ll << 60;
for (int i = 0; i < n; ++i)
ans = min(ans, max(pre[i], suf[i]));
cout << ans << endl;
}
