#include <bits/stdc++.h>
#ifndef dbg
#define dbg(...)
#endif
#define all(x) begin(x), end(x)
#define rsz(...) resize(__VA_ARGS__)
#define psh(...) push_back(__VA_ARGS__)
#define emp(...) emplace_back(__VA_ARGS__)
#define prt(...) print(cout, __VA_ARGS__)
#define dprt(...) dbg(print(cerr,__VA_ARGS__))
#define dmp(...) dprt(#__VA_ARGS__, '=', __VA_ARGS__)
using namespace std;using ll=long long;
template<typename t>using V=vector<t>;
template<typename t>void print(ostream& os, const t& a){os<<a<<'\n';}
template<typename t, typename... A>void print
(ostream& os, const t& a, A&&... b){os<<a<<' ';print(os, b...);}
int main()
{
ios::sync_with_stdio(false);
cin.tie(nullptr);
int x; cin >> x;
int cnt = x;
int n; cin >> n;
for (int i = 0; i < n; ++i)
{
int p; cin >> p;
cnt -= p;
cnt += x;
}
prt(cnt);
}
