#include <bits/stdc++.h>
#define endl "\n"
#define sz = s.size()
using ll = long long;
const int Z = 1e6+9;
const int INF = 1e9+9;
const int mod = 1e9+7;
using namespace std;
ll fp (ll x , ll y){
if (y == 0) return 1;
if (y == 1) return x;
ll ret = fp(x , y/2) % mod;
if (y % 2) ret *= x;
return ret;
}
ll x , n , mn , mx , sum , ans , dif , t , k ;
int main(){
ios::sync_with_stdio(0);cin.tie(NULL);
cin >> x >> n;
x *= (n+1);
for (int i = 0; i < n; i++){
cin >> ans ;
x -= ans;
}
cout << x << endl ;
return 0 ;
}
