#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <iostream>
#include <algorithm>
#include <queue>
#include <map>
#include <set>
#include <vector>
#include <string>
#include <stack>
#include <numeric>
#include <cassert>
#define endl '\n'
#define sp ' '
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
int main(){
ios_base::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);cerr.tie(nullptr);
int x,n;cin>>x>>n;
int ans = x;
while(n--){
int cur;
cin>>cur;
ans -= cur;
ans += x;
}
cout << ans << endl;
}