#include <bits/stdc++.h>
using namespace std;
#define int long long
#define all(x) (x).begin(), (x).end()
#define f first
#define s second
#define yes cout<<"YES\n"
#define no cout<<"NO\n"
#define imp cout<<-1<<"\n"
#define pb push_back
#define pii pair<int,int>
#define piii pair<int,pair<int,int>>
#define fast ios::sync_with_stdio(false);cin.tie(0); cout.tie(0)
const int INF = 1e18;
const int mod=998244353;
void solve(){
int n,k,m,a,b,c;
cin>>n;
vector<int>v(n);
for(int i=0;i<n;i++){
cin>>v[i];
}
vector<int>p(n);
vector<int>s(n);
int mx=0;
p[0]=0;
// cout<<0 <<" ";
for(int i=1;i<n;i++){
p[i]+=p[i-1]+max(v[i-1]-v[i]+1,0LL);
// cout<<p[i]<<" ";
}
s[n-1]=0;
// cout<<'\n';
for(int i=n-2;i>=0;i--){
s[i]+=s[i+1]+max(v[i+1]-v[i]+1,0LL);
}
int ans=1e18;
for(int i=0;i<n;i++){
ans=min(ans,p[i]+max(s[i]-p[i],0LL));
}
cout<<ans<<"\n";
}
signed main() {
fast;
int t=1;
// cin>>t;
solve();
}
