fliperamadorafaik.cpp

#include<bits/stdc++.h>
using namespace std;
const int maxn = 1e5+10;
int n, a[maxn];

int main() {
	// freopen("in.in","r",stdin);
	cin >> n;
	for(int i = 1; i <= n; i++) {
		cin >> a[i];
	}

	int l = 1;
	int r = n;
	long long ans = 0;
	while(l+1 < r) {
		if(a[l] <= 0) {
			ans++;
			l++;
		}
		else if(a[r] <= 0) {
			ans++;
			r--;
		}
		else {
			int mn = min(a[l],a[r]);
			a[l]-=mn;
			a[r]-=mn;
			ans+= mn;
		}
	}


	if(l+1 == r) {
		int mn = min(a[l],a[r]);
		a[l]-=mn;
		a[r]-=mn;
		ans+= mn;

		if(a[l] == 0 && a[r] == 0) {
			cout << ans << endl;
			return 0;
		}
		else if(a[r] == 0) {
			ans++;
			r--;
		}
		else if(a[l] == 0) {
			ans++;
			l++;
		}
	}

	if(a[l]%2 == 1) cout << -1 << endl;
	else {
		ans+= a[l]/2;
		cout << ans << endl;
	}

}