#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
#define ll long long
#define int long long
#define pb push_back
#define fi first
#define se second
#define lf (id<<1)
#define rg ((id<<1)|1)
#define md ((l+r)>>1)
#define ld long double
using namespace std;
typedef pair<int,int> pii;
typedef pair<pii, int> ipii;
const int MAXN = 2e5+10;
const int MAXA = 2e3+10;
const int INF = 1e18+10;
const int LOG = 29;
const int MOD = 1e9+7;
const int SQRT = 450;
const vector<int> NOL = {};
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const vector<int> dx = {1, -1, 0, 0};
const vector<int> dy = {0, 0, 1, -1};
int n;
int a[MAXN], pr[MAXN], su[MAXN];
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin >> n;
for(int i=1; i<=n; i++) cin >> a[i];
for(int i=1; i<=n; i++){
pr[i] = pr[i-1] + max(0ll, a[i-1]-a[i]+1);
}
for(int i=n; i>=1; i--){
su[i] = su[i+1] + max(0ll, a[i+1]-a[i]+1);
}
int ANS = INF;
for(int i=1; i<=n; i++){
ANS = min(ANS, max(pr[i], su[i]));
}
cout << ANS << '\n';
}
