#include <bits/stdc++.h>
#define pb push_back
#define F first
#define S second
#define rep(X, a,b) for(int X=a;X<b;++X)
#define ALL(a) (a).begin(), (a).end()
#define SZ(a) (int)(a).size()
#define NL "\n"
using namespace std;
typedef pair<long long,long long> pll;
typedef pair<int,int> pii;
typedef long long ll;
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << "," << p.second << ')'; }
template<typename A> ostream& operator<<(ostream &os, const vector<A> &p){
for(const auto &a:p)
os << a << " ";
os << "\n";
return os;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n;
cin>>n;
vector<ll> num(n), lft(n), mx(n);
rep(i,0,n) cin>>num[i];
lft[0]=0;
ll cur=0;
rep(i,1,n){
if(num[i]<=num[i-1]){
cur+=num[i-1]-num[i]+1;
}
lft[i]=cur;
}
//cout<<lft;
cur=0;
ll ans=lft[n-1];
for(int i=n-2;i>=0;--i){
if(num[i]<=num[i+1]) cur+=num[i+1]-num[i]+1;
ans=min(ans, max(lft[i], cur));
//cout<<cur<<NL;
}
cout<<ans<<NL;
}
