import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
	MyScanner sc = new MyScanner();
	Scanner sc2 = new Scanner(System.in);
	final int MOD = 1000000007;
	int[] dx = { 1, 0, 0, -1 };
	int[] dy = { 0, 1, -1, 0 };

	void run() {
		while (sc2.hasNext()) {
			int n = sc2.nextInt();
			int r = sc2.nextInt();
			int[] cnt = new int[r];
			for (int i = 0; i < r; i++) {
				cnt[i] = sc2.nextInt();
			}
			ArrayList<Integer> list = new ArrayList<Integer>();
			for (int i = 0; i < n; i++) {
				list.add(i);
			}
			for (int i = 0; i < r; i++) {
				int c = cnt[i];
				ArrayList<Integer> liA = new ArrayList<Integer>();
				ArrayList<Integer> liB = new ArrayList<Integer>();
				for (int j = n / 2; j < n; j++) {
					liA.add(list.get(j));
				}
				for (int j = 0; j < n / 2; j++) {
					liB.add(list.get(j));
				}
				ArrayList<Integer> next = new ArrayList<Integer>();
				while (!liA.isEmpty() || !liB.isEmpty()) {
					int nn = c;
					while (!liA.isEmpty() && nn-- > 0) {
						next.add(liA.get(0));
						liA.remove(0);
					}
					nn = c;
					while (!liB.isEmpty() && nn-- > 0) {
						next.add(liB.get(0));
						liB.remove(0);
					}
				}
				list = next;
			}
			System.out.println(list.get(n - 1));
		}
	}

	public static void main(String[] args) {
		new Main().run();
	}

	void debug(Object... o) {
		System.out.println(Arrays.deepToString(o));
	}

	void debug2(int[][] array) {
		for (int i = 0; i < array.length; i++) {
			for (int j = 0; j < array[i].length; j++) {
				System.out.print(array[i][j]);
			}
			System.out.println();
		}
	}

	boolean inner(int h, int w, int lim) {
		return 0 <= h && h < lim && 0 <= w && w < lim;
	}

	void swap(int[] x, int a, int b) {
		int tmp = x[a];
		x[a] = x[b];
		x[b] = tmp;
	}

	// find minimum i (a[i] >= border)
	int lower_bound(int a[], int border) {
		int l = -1;
		int r = a.length;

		while (r - l > 1) {
			int mid = (l + r) / 2;
			if (border <= a[mid]) {
				r = mid;
			} else {
				l = mid;
			}
		}
		// r = l + 1
		return r;
	}

	class MyScanner {
		int nextInt() {
			try {
				int c = System.in.read();
				while (c != '-' && (c < '0' || '9' < c))
					c = System.in.read();
				if (c == '-')
					return -nextInt();
				int res = 0;
				do {
					res *= 10;
					res += c - '0';
					c = System.in.read();
				} while ('0' <= c && c <= '9');
				return res;
			} catch (Exception e) {
				return -1;
			}
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		String next() {
			try {
				StringBuilder res = new StringBuilder("");
				int c = System.in.read();
				while (Character.isWhitespace(c))
					c = System.in.read();
				do {
					res.append((char) c);
				} while (!Character.isWhitespace(c = System.in.read()));
				return res.toString();
			} catch (Exception e) {
				return null;
			}
		}

		int[] nextIntArray(int n) {
			int[] in = new int[n];
			for (int i = 0; i < n; i++) {
				in[i] = nextInt();
			}
			return in;
		}

		int[][] nextInt2dArray(int n, int m) {
			int[][] in = new int[n][m];
			for (int i = 0; i < n; i++) {
				in[i] = nextIntArray(m);
			}
			return in;
		}

		double[] nextDoubleArray(int n) {
			double[] in = new double[n];
			for (int i = 0; i < n; i++) {
				in[i] = nextDouble();
			}
			return in;
		}

		long[] nextLongArray(int n) {
			long[] in = new long[n];
			for (int i = 0; i < n; i++) {
				in[i] = nextLong();
			}
			return in;
		}
	}
}

Measured memory_bytes: 31360