#include <vector>
#include <algorithm>
#include <iostream>
#include <set>
#include <cmath>
#include <map>
#include <random>
#include <cassert>
#include <ctime>
#include <bitset>
#include <stack>
#include <cstdlib>
#include <queue>
#include <stdint.h>
#include <vector>
#include <cassert>
#include <numeric>
#include <iostream>
#include <algorithm>
#include <functional>
#include <cstdio>
#include <limits.h>
using namespace std;
const int MOD = 1e9 + 7;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int64_t x, n;
cin >> x >> n;
int64_t sum = 0;
for (int i = 0; i < n; i++) {
sum += x; int64_t a; cin >> a; sum -= a;
}
cout << sum + x << '\n';
}
