#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const double eps = 1e-9;
const long long INF = 0x3f3f3f3f3f3f3f3fll;
const int inf = 0x3f3f3f3f;
inline int read() {
  int ret = 0, sgn = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') sgn = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    ret = ret * 10 + ch - '0';
    ch = getchar();
  }
  return ret * sgn;
}
inline void Out(int a) {
  if (a > 9) Out(a / 10);
  putchar(a % 10 + '0');
}
long long __gcd(long long a, long long b) {
  return b == 0 ? a : __gcd(b, a % b);
}
long long lcm(long long a, long long b) { return a * b / __gcd(a, b); }
long long qpow(long long x, long long n, long long mod) {
  long long res = 1;
  while (n) {
    if (n & 1) res = (res * x) % mod;
    x = x * x % mod, n >>= 1;
  }
  return res;
}
const int N = 2e5 + 10;
int main() {
  int n, m;
  int sum = 0;
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) {
    int x;
    scanf("%d", &x);
    sum += x;
  }
  if (sum > n)
    puts("0");
  else
    printf("%d\n", n - sum);
  return 0;
}
