//#pragma GCC optomize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC optimize("O3")
//#pragma GCC target("avx2")
//#pragma GCC target("sse,sse2,sse4.1,sse4.2")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define max_heap priority_queue<pair <ll, pair <ll, ll>>>
#define min_heap priority_queue<pair <ll, ll>, vector<pair <ll, ll>>, greater<pair <ll, ll>>>
#define sariE cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(false);
#define filE freopen("in.txt", "r", stdin); freopen("out1.txt", "w", stdout);
#define endl '\n'
#define md(a) (a % mod + mod) % mod
#define pb push_back
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//cout << setprecision(5) << fixed << f;
//hash prime = 769
ll const maxn = 2e5 + 123;
ll const inf = 2e18;
ll const loG = 23;
//ll const mod = 1e9 + 7;
ll const mod = 998244353;
ll const sq = 500;
ll power(ll a, ll b, ll mod){if(b==0)return 1;if(b==1)return a;ll x = power(a, b / 2, mod);return (((x * x) % mod) * (b % 2 ? a : 1)) % mod;}
ll n, arr[maxn], dp[maxn], pd[maxn];
void Solve(){
cin >> n;
for (int i = 1; i < n + 1; i++)
cin >> arr[i];
for (int i = 1; i < n + 1; i++){
dp[i] = dp[i - 1];
if (arr[i] > arr[i - 1])
continue;
ll diff = arr[i - 1] - arr[i];
dp[i] += diff + 1;
}
for (int i = n; i > 0; i--){
pd[i] = pd[i + 1];
if (arr[i] > arr[i + 1])
continue;
ll diff = arr[i + 1] - arr[i];
pd[i] += diff + 1;
}
// for (int i = 1; i < n + 1; i++)
// cout << i << ' ' << dp[i] << ' ' << pd[i] << endl;
ll ans = min(dp[n], pd[1]);
for (int i = 1; i < n + 1; i++){
ans = min(ans, dp[i] + pd[i] - min(dp[i], pd[i]));
}
cout << ans << endl;
}
int main(){
sariE;// filE;
int test = 1;
//cin >> test;
while (test--) Solve();
return 0;
}
