#include<bits/stdc++.h>
#define F first
#define S second
#define MAX 500005
#define oo 1e18
#define mod 1000000007
#define fast_in ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);cout.setf(ios::fixed);cout.precision(0);
using namespace std;
typedef long long ll;
#define pll pair<ll , ll>
#define vll vector<ll>
#define vvll vector<vll>
#define vpll vector<pll>
ll n,t[MAX],a[MAX],b[MAX];
int main(){
cin>>n;
for(int i=0;i<n;i++)cin>>a[i];
t[n-1]=0;
for(int i=n-2;i>=0;i--){
t[i]=t[i+1]+max((ll)0,(a[i+1]+1)-a[i]);
}ll s=0;ll ans=oo;
for(int i=0;i<n;i++){
if(i>0)s+=max((ll)0,(a[i-1]+1)-a[i]);
ans=min(ans,max(s,t[i]));
}cout<<ans<<"\n";
return 0;
}
