//
//  main.cpp
//  Sort Binary Array
//
//  Created by Himanshu on 06/04/22.
//

#include <iostream>
using namespace std;

int solve (int arr[], int n) {
    int ans = 0;
    int currOnePos = n-1;
    
    for (int index=n-1; index>=0; index--) {
        if (arr[index] == 1) {
            ans += currOnePos-index;
            currOnePos--;
        }
    }
    return ans;
}

int main() {
    
    int n;
    cin>>n;
    
    int *arr = new int[n]();
    
    for (int i=0; i<n; i++) {
        cin>>arr[i];
    }
    
    cout<<solve(arr, n)<<endl;
    
    return 0;
}