// Source: https://usaco.guide/general/io

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

int main() {
	int n;
    cin >> n;

    int curr;
    
    int mainTrackLength = 0;
    int sideTrackLength = 0;

    int firstSideTracker;

    bool hasSeenOutsider = false;
    bool isSideTrackValid = true;

    for(int i = 0; i < n; i++) {
        cin >> curr;

        if (curr == mainTrackLength + 1) {
            mainTrackLength++;
        }
        else if (sideTrackLength == 0) {
            sideTrackLength = 1;
            firstSideTracker = curr;
        }
        else if (curr == firstSideTracker + sideTrackLength) {
            if (!hasSeenOutsider) {
                sideTrackLength++;
            }
        }
        else {
            hasSeenOutsider = true;
            if (curr < firstSideTracker) {
                isSideTrackValid = false;
            }
        }
    }

    int result;

    if (isSideTrackValid && sideTrackLength > 0) {
        result = firstSideTracker + sideTrackLength - 1;
    }
    else {
        result = mainTrackLength;
    }

    cout << result;
}