#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define mp make_pair
#define pii pair<int,int>
#define ss second
#define ff first
#define piii pair<int,pii>
#define debu(x) (cerr << #x << " = "<< x << "\n")
#define debu2(x,y) (cerr << #x << " = "<< x << " " << #y << " = " << y << "\n")
#define debu3(x,y,z) (cerr << #x << " = "<< x << " " << #y << " = " << y << " " << #z << " = " << z<< "\n")
#define bitout(x,y) {\
cerr << #x << " : ";\
for (int justforbits = y; justforbits >=0; justforbits--)cout << (((1 << justforbits) & x)>=1);\
cout << "\n";\
}
#define rangeout(j,rangestart,rangeend) {\
cerr << "outputting" << #j<< ":\n";\
for (int forrang = rangestart; forrang <= rangeend; forrang++)cerr << j[forrang] << " ";\
cerr<<"\n";\
}
#define c1 {cerr << "Checkpoint 1! \n\n";cerr.flush();}
#define c2 {cerr << "Checkpoint 2! \n\n";cerr.flush();}
#define c3 {cerr << "Checkpoint 3! \n\n";cerr.flush();}
#define c4 {cerr << "Checkpoint 4! \n\n";cerr.flush();}
signed main()
{
int t1,t2,t3,t4;
mt19937_64 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());
//ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int n;cin>>n;
vector<int>nummys(n+1);
for(int a=1;a<=n;a++)
{
cin>>nummys[a];
}
int ans=LLONG_MAX;
int currbef=0,curraft=0;
for(int a=1;a<n;a++)
{
curraft+=max(0ll,(nummys[a+1]+1)-nummys[a]);
}
ans=min(ans,curraft);
for(int a=2;a<=n;a++)
{
curraft-=max(0ll,(nummys[a]+1)-nummys[a-1]);
currbef+=max(0ll,(nummys[a-1]+1)-nummys[a]);
ans=min(ans,max(curraft,currbef));
}
cout<<ans;
}
