/** gnu specific **/
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
/** contains everything I need in std **/
#include <bits/stdc++.h>

#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(S) ((int)S.size())
#define FOR(i, n) for(int i = 0; i < n; ++i)
#define RFOR(i, n) for(int i = n-1; i >= 0; --i)
#define output_vec(vec) { FOR(i_, sz(vec)) cout << vec[i_] << ' '; cout << '\n'; }
#define x first
#define y second
#define pb push_back
using namespace std;
typedef long long ll;
typedef vector<ll> vll;
typedef unsigned long long ull;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
typedef pair<double, double> pdd;
typedef vector<bool> vb;
typedef short si;
typedef unsigned long long ull;
typedef long double LD;
typedef pair<ull, ull> pull;
using namespace __gnu_pbds;
typedef tree<ll, null_type, less<>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
using namespace std;
#ifdef ONPC
mt19937 rnd(228);
#else
mt19937 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());
#endif

int n;

ll get_ans(vector<ll>& a)
{
    ll ans = 0;
    FOR(i, n-1)
    {
        if(a[i] > a[i+1])
        {
            if(a[i] - a[i+1] > 1)
            {
                ll delta = a[i]-a[i+1]-1;
                ans += delta;
                a[i] -= delta;
            }
        }
        else
        {
            if(a[i+1] - a[i] > 1)
            {
                ll delta = a[i+1]-a[i]-1;
                ans += delta;
                a[i+1] -= delta;
            }
        }
    }
    return ans;
}

bool is_smooth(const vector<ll>& a)
{
    FOR(i, n-1)
    {
        if(abs(a[i]-a[i+1]) > 1)
            return false;
    }
    return true;
}

int solve()
{
    if (!(cin >> n))
        return 1;
        
    vector<ll> a(n);

    FOR (i, n) cin >> a[i];
    ll ans = get_ans(a);
    reverse(all(a));
    ans += get_ans(a);
    assert(is_smooth(a));

    cout << ans << '\n';
    return 0;
}

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    int TET = 1;
    //cin >> TET;
    for (int i = 1; i <= TET; i++)
    {
        if (solve())
        {
            break;
        }
#ifdef ONPC
        cout << "__________________________" << endl;
#endif
    }
#ifdef ONPC
    cerr << endl << "finished in " << clock() * 1.0 / CLOCKS_PER_SEC << " sec" << endl;
#endif
}