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

int main() {
    int n, m;
    cin >> n >> m;
    int a[m];
    for (int i = 0; i < m; i++) cin >> a[i];
    int ans = 1;
    for (int i = m-1; i >= 0; i--) {
        if (ans == 1) ans = a[i];
        else if (a[i] >= ans) ans--;
    }
    cout << ans << endl;
    return 0;
}