#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>

using namespace std;

int b, p, x;

int main() {
    cin >> b >> p;
    int min = 0;
    while (p-- > 0) {
        cin >> x;
        if (p == 0) {
            if (x > b / 2) min += b - x;
        } else 
        if (x > (b - 1) / 2) min += b - 1 - x;
        else min += x;
    }
    cout << min / 2 +1 << endl;
    return 0;
}