#include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <string>
#include <cstring>
#include <iterator>
#include <random>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef long double ld;
const ll dim = 1e6+7;
//const ll mod = 1e9 + 7;
const ll inf = 1e18 + 77;
#define endl "\n"
#define fi first
#define pb push_back
#define se second
#define vll vector<ll>
ll n, a[dim];
ll pr[dim], sf[dim];
int main(){
ll l, r;
cin>>n;
for(int i=1; i<=n; i++){
cin>>a[i];
}
for(int i=1; i<=n; i++){
pr[i]=pr[i-1];
if(a[i]<=a[i-1]){
pr[i]+=abs(a[i]-a[i-1])+1;
}
}
a[n+1]=0;
for(int i=n; i>=1; i--){
sf[i]=sf[i+1];
if(a[i]<=a[i+1]){
sf[i]+=abs(a[i]-a[i+1])+1;
}
}
ll ans=inf;
for(int i=1; i<=n; i++){
ans=min(ans, max(sf[i], pr[i]));
// cout<<sf[i]<<" "<<pr[i]<<endl;
}
cout<<ans<<endl;
return 0;
}
