#include <bits/stdc++.h>
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
*/
using namespace std;
// using namespace __gnu_pbds;
using namespace chrono;
// mt19937 rng((int) std::chrono::steady_clock::now().time_since_epoch().count());
/*
template <class T> using ordered_set = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
*/
//***************** CONSTANTS *****************
//***************** GLOBAL VARIABLES *****************
//***************** AUXILIARY STRUCTS *****************
//***************** MAIN BODY *****************
void solve(){
int N;
cin >> N;
int A[N + 2];
for(int i = 1; i <= N; i++)
cin >> A[i];
A[0] = A[N + 1] = 0;
int64_t pref[N + 2], suf[N + 2];
// pref[i] -> min number required to fill till ith index from the 1st index
// suf[i] -> min number required to fill till ith index from the nth index
pref[0] = 0;
for(int i = 1; i <= N; i++)
pref[i] = pref[i - 1] + max(0, A[i - 1] - A[i] + 1);
suf[N + 1] = 0;
for(int i = N; i >= 1; --i)
suf[i] = suf[i + 1] + max(0, A[i] - A[i + 1] + 1);
int64_t ans = 1'000'000'000'000'000'000;
for(int i = 0; i <= N; i++)
ans = min(ans, max(pref[i], suf[i + 1]));
cout << ans << '\n';
}
//***************** *****************
int32_t main(){
ios_base::sync_with_stdio(NULL);
cin.tie(NULL);
#ifdef LOCAL
auto begin = high_resolution_clock::now();
#endif
int tc = 1;
// cin >> tc;
for (int t = 0; t < tc; t++)
solve();
#ifdef LOCAL
auto end = high_resolution_clock::now();
cout << fixed << setprecision(4);
cout << "Execution Time: " << duration_cast<duration<double>>(end - begin).count() << "seconds" << endl;
#endif
return 0;
}
/*
If code gives a WA, check for the following :
1. I/O format
2. Are you clearing all global variables in between tests if multitests are a thing
3. Can you definitively prove the logic
*/
