#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ldb;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef pair<ldb,ldb> pdd;
#define ff(i,a,b) for(int i = a; i <= b; i++)
#define fb(i,b,a) for(int i = b; i >= a; i--)
#define trav(a,x) for(auto& a : x)
#define sz(a) (int)(a).size()
#define fi first
#define se second
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
const int mod = 1000000007;
const ll inf = 1e18 + 5;
const int mxN = 200005;
int n;
ll niz[mxN];
ll pref[mxN];
ll sufi[mxN];
int main(){
cin.tie(0)->sync_with_stdio(0);
cin >> n;
ff(i,1,n)cin >> niz[i];
ff(i,1,n)pref[i] = pref[i - 1] + max(0ll, niz[i - 1] - niz[i] + 1);
fb(i,n,1)sufi[i] = sufi[i + 1] + max(0ll, niz[i + 1] - niz[i] + 1);
ll rez = inf;
ff(i,1,n)rez = min(rez, max(pref[i], sufi[i]));
cout << rez << '\n';
return 0;
}
/*
5
3 2 2 3 1
5
9 7 5 3 1
2
2021 2021
8
12 2 34 85 4 91 29 85
// probati bojenje sahovski
*/
