#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll INF = 1e9+7;
const ll MOD = 998244353;
typedef pair<ll,ll> Ii;
#define iii pair<ll,ii>
#define ull unsigned ll
#define f(i,a,b) for(ll i = a;i < b;i++)
#define pb push_back
#define vll vector<ll>
#define F first
#define S second
#define all(x) (x).begin(), (x).end()
///I hope I will get uprating and don't make mistakes
///I will never stop programming
///sqrt(-1) Love C++
///Please don't hack me
///@TheofanisOrfanou Theo830
///Think different approaches (bs,dp,greedy,graphs,shortest paths,mst)
///Stay Calm
///Look for special cases
///Beware of overflow and array bounds
///Think the problem backwards
///Training
int main(void){
ios_base::sync_with_stdio(0);
cin.tie(0);
ll n;
cin>>n;
ll arr[n];
ll b[n];
f(i,0,n){
cin>>arr[i];
}
ll ans = 1e18;
ll L[n] = {0},R[n] = {0};
f(i,0,n-1){
b[i] = arr[i+1] - arr[i];
ll posa = 0;
if(b[i] <= 0){
posa = -b[i] + 1;
}
L[i+1] = L[i] + posa;
}
for(ll i = n-2;i >= 0;i--){
ll posa = 0;
if(b[i] >= 0){
posa = b[i] + 1;
}
R[i] = R[i+1] + posa;
}
f(i,0,n - 1){
ans = min(ans,max(L[i],R[i+1]));
}
cout<<ans<<"\n";
}
