#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
//#include<bits/extc++.h>
//__gnu_pbds
int main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
int n;cin>>n;
vector<int> arr;
int sum = 0;
for(int i=0;i<n;i++){
int a;cin>>a;
arr.push_back(a-sum);
sum = a;
}
ll l=0;ll r = 0;
for(int i=0;i<n;i++){
if(arr[i]>=0) r+=arr[i]+1;
}
ll minn = max(l,r);
for(int i=0;i<n;i++){
if(arr[i]<=0){
l+=1-arr[i];
}
if(arr[i]>=0){
r-=arr[i]+1;
}
minn = min(minn,max(l,r));
}
cout<<minn<<"\n";
return 0;
}
