// #pragma GCC target ("avx2")
// #pragma GCC optimization ("O3")
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
const int MOD = 1e9 + 7;
ll binpow(ll a, ll p, int mod = MOD) {
ll res = 1;
while (p) {
if (p & 1) {
(res *= a) %= mod;
}
p >>= 1;
(a *= a) %= mod;
}
return res;
}
ll gcd(ll a, ll b) {
return b == 0 ? a : gcd(b, a % b);
}
void solve() {
int n;
cin >> n;
vector<int> b;
int last = 0;
ll left = 0, right = 0;
for (int i = 0; i < n; i++) {
int a;
cin >> a;
if (i) b.push_back(a - last);
last = a;
if (i && b.back() >= 0) right += b.back() + 1;
}
ll ans = right;
for (int i = 0; i + 1 < n; i++) {
if (b[i] >= 0) right -= b[i] + 1;
if (b[i] <= 0) left -= b[i] - 1;
ans = min(ans, max(left, right));
}
cout << ans << '\n';
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int T = 1;
// cin >> T;
for (int tc = 1; tc <= T; tc++) {
// cout << "Case #" << tc << ": ";
solve();
}
return 0;
}
