import java.util.*;

public class Main {
	public static void main (String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int n = sc.nextInt();
    	TreeMap<Integer, Integer> stock = new TreeMap<>();
    	ArrayDeque<Integer> zeros = new ArrayDeque<>();
    	long total = 0;
    	for (int i = 0; i < n; i++) {
    	    int x = sc.nextInt();
    	    if (x == 0) {
    	        if (stock.size() == 0) {
    	            zeros.push(i);
    	        } else {
    	            int key = stock.lastKey();
    	            total += i - key;
    	            if (stock.get(key) == 1) {
    	                stock.remove(key);
    	            } else {
    	                stock.put(key, stock.get(key) - 1);
    	            }
    	        } 
    	    } else if (x > 1) {
    	        for (int j = 1; j < x; j++) {
    	            if (zeros.size() == 0) {
    	                stock.put(i, x - j);
    	                break;
    	            } else {
    	                total += i - zeros.pop();
    	            }
    	        }
    	    }
    	}
    	System.out.println(total);
    }
}