#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pii;
#define FOR(i, j, n) for(ll i = j; i<= n; i++)
#define ROF(i, n, j) for(ll i = n; i>= j; i--)
#define pb push_back
#define F first
#define S second
#define IOS ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define G(i, j) get<j-1>(i)
#define prll(x) cout << #x << ": " << x << endl;
const ll mn = 2e5 + 5, inf = 1e18;
ll a[mn], b[mn];
signed main()
{
IOS;
ll n;
cin >> n;
FOR(i,1 ,n) cin>> a[i];
FOR(i, 1, n+1)
{
b[i] = a[i]-a[i-1];
}
ll l = 0, r = 0;
FOR(i, 2, n+1)
{
if (b[i] >= 0) r += b[i]+1;
}
ll ans = max(l, r);
FOR(i, 2, n)
{
if (b[i] >= 0) r -= b[i]+1;
if (b[i] <= 0) l += (-b[i])+1;
ans = min(ans, max(l, r));
}
cout << ans;
return 0;
}
