#include <bits/stdc++.h>
using namespace std;
int nextInt() {
  int x;
  scanf("%d", &x);
  return x;
}
long long nextLong() {
  long long x;
  scanf("%I64d", &x);
  return x;
}
double nextDouble() {
  double x;
  scanf("%lf", &x);
  return x;
}
const int BUFSIZE = 1000000;
char buf[BUFSIZE + 1];
string nextString() {
  scanf("%s", buf);
  return buf;
}
struct Point {
  double x, y;
  Point() : x(0), y(0) {}
  Point(double x, double y) : x(x), y(y) {}
  Point operator-(Point op) const { return Point(x - op.x, y - op.y); }
  Point operator+(Point op) const { return Point(x + op.x, y + op.y); }
  Point operator*(double op) const { return Point(x * op, y * op); }
  double operator*(Point op) const { return x * op.x + y * op.y; }
  double operator%(Point op) const { return x * op.y - y * op.x; }
  double length2() { return x * x + y * y; }
  double lengt() { return sqrt(length2()); }
};
Point nextPoint() {
  double x = nextDouble();
  double y = nextDouble();
  return Point(x, y);
}
int main() {
  int d = nextInt();
  int n = nextInt();
  int sum = 0;
  for (int i = 0; i < n - 1; ++i) {
    sum += d - nextInt();
  }
  cout << sum << endl;
  return 0;
}
