#include <bits/stdc++.h>
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ull, ull> pull;
typedef pair<ll, ll> pii;
typedef pair<ld, ld> pld;
ll x, n;
ll a[1009];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
//freopen("in.txt", "r", stdin);
//freopen("out.txt", "w", stdout);
cin >> x >> n;
for(ll i = 0; i < n; ++i)
cin >> a[i];
ll ans = 0;
for(ll i = 0; i < n; ++i)
ans += x-a[i];
ans += x;
cout << ans << "\n";
}
