#include<bits/stdc++.h>
using namespace std;
using ll=long long;
//#pragma GCC optimize("03,unroll-loops")
//#pragma GCC target("avx2")
//#pragma GCC target("sse4")
#define all(v) v.begin(),v.end()
#define F first
#define S second
#define pub push_back
#define pob pop_back
#define puf push_front
#define pof pop_front
//#define randi uniform_int_distribution<long long>
#define damoon(v) v.resize(unique(all(v))-v.begin())
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//randi dist(0,10000000000000000);
typedef pair<int,int> pii;
typedef pair<long long,long long> pll;
typedef pair<int,bool> pib;
typedef pair<long long,bool> plb;
typedef pair<int,pii> pip;
typedef pair<pii,int> ppi;
typedef vector<int> veci;
typedef vector<long long> vecl;
typedef vector<bool> vecb;
typedef vector<pii> vecp;
typedef set<int> seti;
typedef set<long long> setl;
typedef set<pii> setp;
typedef map<int,int> mapii;
typedef map<long long,long long> mapll;
typedef map<int,bool> mapib;
typedef map<long long,bool> maplb;
const int inf=1e9,mod=1e9+7,neginf=-1e9;
const double PI=acos(-1);
void solve()
{
int n;
cin>>n;
vecl a(n+2);
vecl dif(n-1),prf(n),suf(n);
for(int i=0;i<n-1;i++)
dif[i]=a[i+1]-a[i];
prf[0]=0;
for(int i=0;i<n-1;i++)
prf[i+1]=prf[i]+max(0LL,1-dif[i]);
suf[n-1]=0;
for(int i=n-2;i>=0;i--)
suf[i]=suf[i+1]+max(0LL,dif[i]+1);
ll ans=LLONG_MAX;
for(int i=0;i<n;i++)
ans=min(ans,max(prf[i],suf[i]));
cout<<ans<<"\n";
}
int main()
{
cin.tie(0);
cout.tie(0);
ios_base::sync_with_stdio(false);
//ifstream fin("in.txt");
//ofstream fout("out.txt");
int t=1;
//cin>>t;
while(t--)
{
solve();
}
}
