#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pi;
typedef pair<ll, int> pli;
typedef pair<int, ll> pil;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;
typedef vector<int> vi;
typedef vector<bool> vb;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<pli> vli;
typedef vector<pil> vil;
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
const int MOD = 1000000007; // 998244353
const ll INF = 1e18;
const int oo = 1000000007;
const int MX = 100001;
const ld PI = 4 * atan((ld)1);
int x, n;
void solve() {
cin >> x >> n;
int sum = x * (n + 1);
for (int i = 1; i <= n; i++) { int x; cin >> x; sum -= x; }
cout << sum;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
// freopen("name.inp", "r", stdin);
// freopen("name.out", "w", stdout);
solve();
return 0;
}
