#include <bits/stdc++.h> using namespace std; #define int long long #define ld long double #define pii pair<int, int> #define fi first #define se second #define __builtin_popcount __builtin_popcountll #define all(x) (x).begin(), (x).end() #define BIT(x, i) (((x) >> (i)) & 1) #define debug(a, l, r) for (int i = (l); i <= (r); ++i) cout << (a)[i] << ' '; cout << '\n'; const int MAXN = 2e5 + 5; const int INF = 1e18 + 5; int n, a[MAXN], pref[MAXN], suff[MAXN]; signed main() { #ifdef NCTHANH freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout); #endif ios_base::sync_with_stdio(0); cin.tie(nullptr); cout.tie(nullptr); cin >> n; for (int i = 1; i <= n; ++i) cin >> a[i]; { pref[1] = 0; for (int i = 2; i <= n; ++i) { int cur = max(0ll, a[i - 1] + 1 - a[i]); pref[i] = pref[i - 1] + cur; } } { suff[n] = 0; for (int i = n - 1; i >= 1; --i) { int cur = max(0ll, a[i + 1] + 1 - a[i]); suff[i] = suff[i + 1] + cur; } } int res = INF; for (int i = 1; i <= n; ++i) res = min(res, max(pref[i], suff[i])); cout << res; return 0; }