import java.util.*;
 
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
 
        // 读取火车数量
        int n = scanner.nextInt();
 
        // 读取火车出站顺序
        int[] departureOrder = new int[n];
        for (int i = 0; i < n; i++) {
            departureOrder[i] = scanner.nextInt();
        }
 
        // 调用方法来获取最大车站内火车数量
        int result = findMaxTrainsAtStation(n, departureOrder);
 
        // 输出结果
        System.out.println(result);
    }
 
    public static int findMaxTrainsAtStation(int n, int[] departureOrder) {
        // 使用栈来模拟车站
        Stack<Integer> stack = new Stack<>();
        int currentTrain = 1;  // 当前到达的火车
        int maxTrains = 0;
 
        // 遍历每一个出站的火车
        for (int i = 0; i < n; i++) {
            // 将火车推入栈中，直到栈顶是当前应该出站的火车
            while (currentTrain <= n && (stack.isEmpty() || stack.peek() != departureOrder[i])) {
                stack.push(currentTrain++);
            }
 
            // 检查栈顶的火车是否是当前应该出站的火车
            if (!stack.isEmpty() && stack.peek() == departureOrder[i]) {
                stack.pop();  // 出站
            } else {
                // 如果栈顶不是当前应该出站的火车，说明顺序不对，返回 -1
                return -1;
            }
 
            // 更新车站内最大火车数量
            maxTrains = Math.max(maxTrains, stack.size());
        }
 
        return maxTrains;
    }
}
 

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