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#define pb push_back
#define mp make_pair
#define fi first
#define se second 
#define all(...) begin(__VA_ARGS__) , end(__VA_ARGS__)
#define boost {ios_base::sync_with_stdio(false); cin.tie(); cout.tie();}

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef vector <int> vi;
typedef pair<int,int> PII;
typedef pair<ll,ll> PLL;
constexpr ll nax = 1e6+6969, INF = 2e16+6969;

ll n,dp[nax],p[2][nax];
vector <PLL> v;

int main()
{
	scanf("%lld", &n);
	int cnt = 0;
	v.pb(mp(0,0));
	for(int i=0;i<n;++i)
	{
		ll x;
		scanf("%lld", &x);
		if(x != 0)
		{
			v.pb(mp(x,cnt));
			cnt = 0;
		}
		cnt++;
	}
	//  for(auto e : v) printf("%d %d\n", e.fi, e.se);
	for(int i=1;i<=n;++i) 
	{
		p[0][i] = p[0][i-1] + v[i].fi;
		p[1][i] = p[1][i-1] + v[i].se;
	}
	dp[0] = 0;
	//  if(v[1].fi > 0) dp[1] = 0;
	//  else dp[1] = INF;
	for(int i=1;i<=n;++i)
	{
		ll best = INF;
		for(int j=0;j<i;++j)
		{
			if(p[0][i] - p[0][j] >= 0)
			{
				best = min(best, p[1][i] - p[1][j+1] + dp[j]);
			}
		}
		dp[i] = best;
	}
	if(dp[n] >= INF) puts("-1");
	else printf("%lld\n", dp[n]); 
	return 0;
}