#include "bits/stdc++.h"

using namespace std;
using ll = long long;
const double pi = acos(-1);
#define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++)
#define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME
#define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__)
#define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__)
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v) {
	REP(i, v.size()) { if (i)os << " "; os << v[i]; }return os;
}
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v) {
	REP(i, v.size()) { if (i)os << endl; os << v[i]; }return os;
}

const ll INF = LLONG_MAX;
ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

#define int long long

inline void my_io() {

	std::ios::sync_with_stdio(false);

	std::cin.tie(0);

	cout << fixed << setprecision(16);

	//cout << setprecision(10) << scientific << ans << endl;
}

signed main() {
	ll n;

	cin >> n;

	vector<ll> v(n);
	ll sum = 0;

	REP(i, n) {
		cin >> v[i];
		sum += v[i];
	}

	ll now = 1;
	ll ma = 0;

	while (true) {
		if (ma + now * 2 > sum) {
			if (ma + now > sum) {
				ma -= now - 1;
				now--;
				break;
			}
			else {
				ma += now;
				break;
			}
		}

		ma += now * 2;
		now++;
	}

	vector<ll> vv;

	REP(i, now) {
		vv.push_back(i + 1);
	}

	RREP(i, now - 1) {
		vv.push_back(i + 1);
	}

	FOR(i, v.size(), vv.size()) {
		v.push_back(0);
	}

	FOR(i, vv.size(), v.size()) {
		vv.push_back(0);
	}

	ll ans = 0;

	REP(i, v.size()) {
		if (v[i] > vv[i]) {
			ans += v[i] - vv[i];
		}
	}

	cout << ans << endl;
}