import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;
import java.util.function.BiFunction;
import java.util.function.Function;
public class Main {
void run() {
int k = ni();
int t = ni();
int[] a = new int[t];
for (int i = 0; i < t; ++i) {
a[i] = ni();
}
debug(a);
int cnt = 0;
int tmp = -1;
boolean flag = false;
for (int i = 0; i < k; ++i) {
int max = 0;
int idx = 0;
for (int j = 0; j < t; ++j) {
if (tmp == j) {
continue;
}
if (max < a[j]) {
max = a[j];
idx = j;
}
}
if (max == 0) {
flag = true;
System.out.println(k - i);
break;
}
a[idx]--;
tmp = idx;
// debug(a, tmp);
}
if (!flag) {
System.out.println(0);
}
}
Scanner sc = new Scanner(System.in);
public static void main(String[] args) {
new Main().run();
}
int ni() {
return Integer.parseInt(sc.next());
}
void debug(Object... os) {
System.err.println(Arrays.deepToString(os));
}
class BIT<T> {
int n;
ArrayList<T> bit;
BiFunction<T, T, T> bif;
BIT(int n, BiFunction<T, T, T> bif, T defaultValue) {
this.n = n;
bit = new ArrayList<>(n + 1);
for (int i = 0; i < n + 1; ++i) {
bit.add(defaultValue);
}
this.bif = bif;
}
void update(int i, T v) {
for (int x = i; x <= n; x += x & -x) {
bit.set(x, bif.apply(bit.get(x), v));
}
}
T reduce(int i, T defaultValue) {
T ret = defaultValue;
for (int x = i; x > 0; x -= x & -x) {
ret = bif.apply(ret, bit.get(x));
}
return ret;
}
}
long MOD = 1_000_000_007;
long pow(long a, long r) {
long sum = 1;
while (r > 0) {
if ((r & 1) == 1) {
sum *= a;
sum %= MOD;
}
a *= a;
a %= MOD;
r >>= 1;
}
return sum;
}
long C(int n, int r) {
long sum = 1;
for (int i = n; 0 < i; --i) {
sum *= i;
sum %= MOD;
}
long s = 1;
for (int i = r; 0 < i; --i) {
s *= i;
s %= MOD;
}
sum *= pow(s, MOD - 2);
sum %= MOD;
long t = 1;
for (int i = n - r; 0 < i; --i) {
t *= i;
t %= MOD;
}
sum *= pow(t, MOD - 2);
sum %= MOD;
return sum;
}
double GOLDEN_RATIO = (1.0 + Math.sqrt(5)) / 2.0;
/**
* 黄金分割探索
*
* @param left 下限
* @param right 上限
* @param f 探索する関数
* @param comp 上に凸な関数を探索するときは、Comparator.comparingDouble(Double::doubleValue)
* 下に凸な関数を探索するときは、Comparator.comparingDouble(Double::doubleValue).reversed()
* @return 極値の座標x
*/
double goldenSectionSearch(double left, double right, Function<Double, Double> f, Comparator<Double> comp) {
double c1 = divideInternally(left, right, 1, GOLDEN_RATIO);
double c2 = divideInternally(left, right, GOLDEN_RATIO, 1);
double d1 = f.apply(c1);
double d2 = f.apply(c2);
while (right - left > 1e-9) {
if (comp.compare(d1, d2) > 0) {
right = c2;
c2 = c1;
d2 = d1;
c1 = divideInternally(left, right, 1, GOLDEN_RATIO);
d1 = f.apply(c1);
} else {
left = c1;
c1 = c2;
d1 = d2;
c2 = divideInternally(left, right, GOLDEN_RATIO, 1);
d2 = f.apply(c2);
}
}
return right;
}
/**
* [a,b]をm:nに内分する点を返す
*/
double divideInternally(double a, double b, double m, double n) {
return (n * a + m * b) / (m + n);
}
}