#include <bits/stdc++.h>
#define debug(x) cerr << #x << " " << x << "\n"
#define debugs(x) cerr << #x << " " << x << " "
using namespace std;
typedef long long ll;
typedef pair <int, int> pii;
typedef pair <long double, pii> muchie;
const ll NMAX = 1000001;
const ll VMAX = 1001;
const ll INF = (1LL << 60);
const ll MOD = 1000000007;
const ll BLOCK = 447;
const ll base = 31;
const ll nr_of_bits = 21;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n, x, i, p;
cin >> x >> n;
x *= (n + 1);
for(i = 1; i <= n; i++){
cin >> p;
x -= p;
}
cout << x;
return 0;
}
