#ifndef LOCAL
#define FAST_IO
#endif

// ============
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define OVERRIDE(a, b, c, d, ...) d
#define REP2(i, n) for (i32 i = 0; i < (i32)(n); ++i)
#define REP3(i, m, n) for (i32 i = (i32)(m); i < (i32)(n); ++i)
#define REP(...) OVERRIDE(__VA_ARGS__, REP3, REP2)(__VA_ARGS__)
#define PER(i, n) for (i32 i = (i32)(n) - 1; i >= 0; --i)
#define ALL(x) begin(x), end(x)

using namespace std;

using u32 = unsigned int;
using u64 = unsigned long long;
using i32 = signed int;
using i64 = signed long long;
using f64 = double;
using f80 = long double;

template <typename T>
using Vec = vector<T>;

template <typename T>
bool chmin(T &x, const T &y) {
    if (x > y) {
        x = y;
        return true;
    }
    return false;
}
template <typename T>
bool chmax(T &x, const T &y) {
    if (x < y) {
        x = y;
        return true;
    }
    return false;
}

template <typename T>
Vec<tuple<i32, i32, T>> runlength(const Vec<T> &a) {
    if (a.empty()) {
        return Vec<tuple<i32, i32, T>>();
    }
    Vec<tuple<i32, i32, T>> ret;
    i32 prv = 0;
    REP(i, 1, a.size()) {
        if (a[i - 1] != a[i]) {
            ret.emplace_back(prv, i, a[i - 1]);
            prv = i;
        }
    }
    ret.emplace_back(prv, (i32)a.size(), a.back());
    return ret;
}

#ifdef INT128

using u128 = __uint128_t;
using i128 = __int128_t;

istream &operator>>(istream &is, i128 &x) {
    i64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, i128 x) {
    os << (i64)x;
    return os;
}
istream &operator>>(istream &is, u128 &x) {
    u64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, u128 x) {
    os << (u64)x;
    return os;
}

#endif

[[maybe_unused]] constexpr i32 INF = 1000000100;
[[maybe_unused]] constexpr i64 INF64 = 3000000000000000100;
struct SetUpIO {
    SetUpIO() {
#ifdef FAST_IO
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
#endif
        cout << fixed << setprecision(15);
    }
} set_up_io;
// ============

#ifdef DEBUGF
#else
#define DBG(x) (void)0
#endif

struct PQ {
    i64 added;
    priority_queue<i64, Vec<i64>> pq;
    
    PQ() : added(0), pq() {}
    
    void add(i64 v) {
        added += v;
    }
    
    void push(i64 v) {
        pq.push(v - added);
    }
    
    i64 max() const {
        return pq.top() + added;
    }
    
    void pop() {
        pq.pop();
    }
    
    bool empty() const {
        return pq.empty();
    }
};

int main() {
    i32 n;
    cin >> n;
    Vec<i32> a(n);
    REP(i, n) {
        cin >> a[i];
    }
    sort(ALL(a));
    REP(i, n) {
        a[i] -= i;
    }
    Vec<i32> b(n - 1);
    REP(i, n - 1) {
        b[i] = a[i] - a[i + 1];
    }
    DBG(b);
    PQ pq;
    i64 f0 = 0;
    REP(i, n - 1) {
        if (pq.empty() || pq.max() <= 0) {
            pq.push(0);
        } else {
            f0 += pq.max();
            pq.pop();
            pq.push(0);
            pq.push(0);
        }
        pq.add(b[i]);
        DBG(f0);
        DBG(pq.added);
        DBG(pq.pq);
    }
    i64 ans = 0;
    chmax(ans, f0);
    chmax(ans, f0 + pq.max());
    cout << ans << '\n';
}