#include <bits/stdc++.h>
#define int long long
using namespace std;
#define F first
#define S second
#define pb push_back
#define ins insert
#define pf push_front
#define lb lower_bound
#define ub upper_bound
#define yes cout << "YES" << endl
#define no cout << "NO" << endl
#define srt(p) sort(p.begin() , p.end())
#define rvr(p) reverse(p.begin() , p.end())
#define all(p) (p.begin() , p.end())
const int INF = 1e6 , MOD = 998244353 , MAX = 1e7;
int dx[8] = {-1 , -1 , -1 , 0 , 0 , 1 , 1 , 1};
int dy[8] = {-1 , 0 , 1 , -1 , 1 , -1 , 0 , 1};
bool is_prime(int n) {
if(n == 1 or n == 0) return false;
if(n == 2) return true;
if(n % 2 == 0) return false;
for(int i = 3; i <= sqrt(n); i += 2) {
if(n % i == 0) return false;
}
return true;
}
struct Edge {
int u , v , w;
};
void solve() {
int n;
cin >> n;
vector<int> vt(n + 1 , 0) , pref(n + 2 , 0) , suff(n + 2 , 0);
for(int i = 1; i <= n; ++ i) cin >> vt[i];
for(int i = 2; i <= n; ++ i) {
pref[i] = pref[i - 1];
if(vt[i] <= vt[i - 1]) pref[i] += (vt[i - 1] - vt[i] + 1);
}
for(int i = n - 1;i >= 1; i--) {
suff[i] = suff[i + 1];
if(vt[i] <= vt[i + 1]) {
suff[i] += (vt[i + 1] - vt[i] + 1);
}
}
int ans = INT_MAX;
for(int i = 1; i <= n; ++ i) ans = min(ans , max(pref[i] , suff[i]));
cout << ans << endl;
}
signed main() {
// freopen("input.txt","r",stdin);
// freopen("output.txt","w",stdout);
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int t = 1;
//cin >> t;
while (t--) {
solve();
}
}
