/*
/^--^\
\____/
/ \ _____ _ __ __ ____ _ ____ ____ _____
| || ()_)| |\ \/ /| ===|| |__ / (__` / () \|_ _|
\__ __/ |_| |_|/_/\_\|____||____|\____)/__/\__\ |_|
|^|^\ \^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|^|
| | |\ \| | | | | | | | | | | | | | | | | | | | | | | | |
#####/ /#################################################
| | |\/ | | | | | | | | | | | | | | | | | | | | | | | | |
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|*/
//#pragma GCC optimize("O4,unroll-loops,no-stack-protector")
#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ull=unsigned long long;
using pii=pair<ll,ll>;
#define int ll
#define double long double
#define For(i,a,b) for(int i=a;i<=b;i++)
#define Forr(i,a,b) for(int i=a;i>=b;i--)
#define F first
#define S second
#define L(id) (id*2+1)
#define R(id) (id*2+2)
#define LO(x) (x&(-x))
#define eb emplace_back
#define all(x) x.begin(),x.end()
#define sz(x) ((int)x.size())
#define mkp make_pair
#define MOD (ll)(1000000007)
#define INF (ll)(1e15)
#define EPS (1e-9)
#ifdef LOCALMEOW
#define debug(...) do{\
cerr << __LINE__ <<\
" : ("#__VA_ARGS__ << ") = ";\
_OUT(__VA_ARGS__);\
}while(0)
template<typename T> void _OUT(T x) { cerr << x << "\n"; }
template<typename T,typename...I>
void _OUT(T x,I ...tail) { cerr << x << ", "; _OUT(tail...); }
#else
#define debug(...)
#endif
inline void IOS(){ ios::sync_with_stdio(false); cin.tie(0); }
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
int gcd(int a,int b) { return b==0?a:gcd(b,a%b); }
int lcm(int a,int b) { return a/gcd(a,b)*b; }
int fpow(int b,int p){
int ans=1,now=b;
while(p){
if(p&1) ans=ans*now%MOD;
p/=2; now=now*now%MOD;
}
return ans;
}
void minify(int &a,int b) { if(b<a) a=b; }
void maxify(int &a,int b) { if(b>a) a=b; }
int32_t main(){
IOS();
//code...
int n; cin>>n;
vector<int> v(n);
for(auto &i:v)
cin>>i;
vector<int> l(n),r(n);
l[0]=0;
For(i,1,n-1) l[i]=l[i-1]+max(0ll,v[i-1]-v[i]+1);
r[n-1]=0;
Forr(i,n-2,0) r[i]=r[i+1]+max(0ll,v[i+1]-v[i]+1);
int ans=INF;
For(i,0,n-1) ans=min(ans,max(r[i],l[i]));
cout<<ans<<"\n";
return 0;
}
