#include<bits/stdc++.h>
#define x first
#define y second
#define pb push_back
#define mp make_pair
#define sz(v) v.size()
#define up_b upper_bound
#define low_b lower_bound
#define all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
typedef long double ld;
const int N=2e5+11;
const ll mod=1e9+7;
const ll INF=1e18;
ll a[N],b[N];
ll pref[N],suf[N];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0),cout.tie(0);
int n;
cin>>n;
for(int i=1;i<=n;i++){
cin>>a[i];
if(i>1)b[i-1]=a[i]-a[i-1];
}
for(int i=1;i<n;i++){
pref[i]=pref[i-1]+max(-b[i]+1,0ll);
}
for(int i=n-1;i>=1;i--){
suf[i]=suf[i+1]+max(b[i]+1,0ll);
}
ll res=INF;
for(int i=1;i<=n;i++){
// cout<<pref[i-1]<<" "<<suf[i]<<endl;
res=min(res,max(pref[i-1],suf[i]));
}
cout<<res;
return 0;
}
