#include <bits/stdc++.h>
using namespace std;
#define int long long
#define FOR(I, L, R) for(int I(L) ; I <= (int)R ; ++I)
#define FOD(I, R, L) for(int I(R) ; I >= (int)L ; --I)
#define FOA(I, A) for(auto &I : A)
#define print(A,L,R) FOR(OK, L, R){if(A[OK]<=-oo / 10||A[OK]>=oo)cout<<"- ";else cout<<A[OK]<<' ';}cout<<'\n';
#define prints(A) FOA(OK, A){cout<<OK<<' ';}cout << '\n';
#define printz(A,L,R) FOR(OK, 1, L){FOR(KO, 0, R){if(A[OK][KO]>-oo&&A[OK][KO]<oo)cout<<A[OK][KO]<<' ';else cout << "- ";} cout << '\n';}cout << '\n';
#define fs first
#define sd second
#define ii pair<int,int>
#define iii pair<int, ii>
#define all(A) A.begin(), A.end()
#define quickly ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define FILE "FILE"
const int N = 3e5 + 5;
const int mod = 1e9 + 9999;
const int oo = 1e9;
struct Modint{
int x;
Modint(){x = 0;}
Modint(int _x){x = (_x + mod) % mod;}
Modint operator + (const Modint &other) const{ return Modint((x + other.x) % mod); }
Modint operator - (const Modint &other) const{ return Modint((x - other.x + mod) % mod); }
Modint operator * (const Modint &other) const{ return Modint((1LL * x * other.x) % mod); }
void operator += (const Modint &other) { *this = *this + other; }
void operator -= (const Modint &other) { *this = *this - other; }
void operator *= (const Modint &other) { *this = *this * other; }
friend ostream& operator << (ostream& os, const Modint &other){
return os << other.x;
}
};
int n;
int a[N];
int pre[N], suf[N], cost[N];
int st[N], en[N];
signed main(){ quickly
if(fopen(FILE".in", "r")){
freopen(FILE".in", "r", stdin);
freopen(FILE".out", "w", stdout);
}
cin >> n;
FOR(i, 1, n){
cin >> a[i];
}
st[1] = 1;
FOR(i, 2, n){
if(a[i] <= a[i - 1]){
st[i] = i;
}
else{
st[i] = st[i - 1];
}
int val = a[st[i] - 1] - a[st[i]] + 1;
cost[i] = val;
pre[i] = pre[st[i] - 1] + val;
}
en[n] = n;
FOD(i, n - 1, 1){
if(a[i] <= a[i + 1]){
en[i] = i;
}
else{
en[i] = en[i + 1];
}
int val = a[en[i] + 1] - a[en[i]] + 1;
cost[i] = min(cost[i], val);
suf[i] = suf[en[i] + 1] + val;
}
int ans = oo;
FOR(i, 1, n){
ans = min(ans, max(pre[i], suf[i]));
}
cout << ans;
}
