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

int main() {
    int n;
    cin >> n;
    vector<long long int> energy(n+1);
    
    for(int i = 1; i <= n; i++) {
        cin >> energy[i];
    }
    
    vector<long long int> prefixSum(n+1);
    for(int i = 1; i <= n; i++) {
        prefixSum[i] = prefixSum[i-1] + energy[i];
    }
    
    vector<long long int> dp(n+1, LONG_LONG_MAX);
    map<long long int, long long int> M;
    dp[0] = 0;
    M[0] = -1;

    for(int i = 1; i <= n; i++) {    
        long long int key = prefixSum[i-1];
        long long int val = dp[i-1] - i;
       
        if(dp[i-1] < LONG_LONG_MAX) {      
            auto it = M.lower_bound(key);
            while(it != M.end()) {
                if(it->second > val) {
                   it = M.erase(it);
               } else {
                   break;
               }
            }
            M[key] = val;
        }
        
        auto ub = M.upper_bound(prefixSum[i]);
        if(ub != M.begin()) {
            ub--;
            if(prefixSum[i] - ub->first >= 0) {
                dp[i] = ub->second + i;         
            }
        }
    }
    dp[n] <= n + 2? cout << dp[n] << endl : cout << -1 << endl;
}