#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define MASK(x) (1LL << (x))
#define BIT(x, i) (((x) >> (i)) & 1)
#define SZ(x) ((int) (x).size())
#define ALL(a) (a).begin(), (a).end()
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define FORD(i, a, b) for (int i = (a); i >= (b); --i)
#define debug(x) cerr << "[" << #x << " = " << (x) << "]" << endl
#define left __left
#define right __right
#define prev __prev
#define fi first
#define se second
template <class X, class Y>
bool maximize(X &x, Y y) {
if (x < y) return x = y, true;
else return false;
}
template <class X, class Y>
bool minimize(X &x, Y y) {
if (x > y) return x = y, true;
else return false;
}
int maxCap, numMonth;
#define MAX_N 102
int a[MAX_N + 2];
int main() {
ios_base::sync_with_stdio(false);cin.tie(nullptr);
cin >> maxCap;
cin >> numMonth;
FOR(i, 1, numMonth) cin >> a[i];
cout << maxCap * (numMonth + 1) - accumulate(a + 1, a + 1 + numMonth, 0);
return 0;
}
/* Discipline - Calm */
