Submission #3570794

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Source Code Expand

// Ala be zekrellah tatmaenolgholoob ...
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define F first
#define S second
#define MP make_pair
const int maxn = 2e5+9;
const ll mod = 1e9+7;

int a[maxn];

int main () {
	ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
		cin >> a[i];
	for (int i = 0; i < n; i++)
		if (a[i] > i)
			return cout << "-1\n", 0;
	int ind = 0;
	while (ind < n - 1 && a[ind] == a[ind + 1] - 1) ind++;
	for (int i = 0; i < n; i++)
		if (a[i] == i && i > ind)
			return cout << "-1\n", 0;
	ll ans = 0;
	ind = n - 1;
	while (true) {
		ans += a[ind];
		while (ind != 0 && a[ind - 1] == a[ind] - 1) ind--;
		if (ind == 0) break;
		ind--;
	}
	cout << ans << "\n";
}

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