#include <bits/stdc++.h>
#define endl '\n'
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define F first
#define S second
#define pf push_front
#define pb push_back
#define ll long long
#define ull unsigned long long
#define ld long double
#define int long long
#define vi vector <int>
#define pii pair <int,int>
#define tiii tuple<int,int,int>
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
#define YES cout << "YES" << endl
#define NO cout << "NO" << endl
#define out(v) for (auto x:v) cout << x << ' '
#define out1(v) for (auto x:v) cout << x << endl
#define inp(arr) for (int i=0;i<arr.size();i++) cin>>arr[i]
const int MAX=1e6;
const int MOD=1e9+7;
using namespace std;
const int inf=1e18;
void solve(){
int n;
cin>>n;
vi arr(n+1),pref(n+1,0),suf(n+1,0);
for(int i=1;i<=n;i++){
cin>>arr[i];
}
for(int i=2;i<=n;i++){
if(arr[i]<=arr[i-1]){
pref[i]=pref[i-1]+arr[i-1]-arr[i]+1;
}
else pref[i]=pref[i-1];
}
for(int i=n-1;i>=1;i--){
if(arr[i]<=arr[i+1]){
suf[i]=suf[i+1]+arr[i+1]-arr[i]+1;
}
else suf[i]=suf[i+1];
}
int ans=inf;
for(int i=1;i<=n;i++){
ans=min(ans,max(suf[i],pref[i]));
}
cout<<ans<<endl;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(nullptr);
int t=1;
//cin>>t;
while(t--) {
solve();
}
}
