#include <bits/stdc++.h>
#define fr(i, n, m) for(int i = (n); i < (m); i ++)
#define pb push_back
#define st first
#define nd second
#define pq priority_queue
#define all(x) begin(x),end(x)
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
ll const inf = 1e9;
ll const mod = 1e9 + 7;
ld const eps = 1e-9;
int main()
{
int x, n;
cin >> x >> n;
int tot = 0;
fr(i, 0, n){
tot += x;
int lost;
cin >> lost;
tot -= lost;
}
tot += x;
cout << tot << endl;
return 0;
}
