/* #pragma GCC optimize("Ofast,unroll-loops") */
#include "bits/stdc++.h"
using namespace std;
#ifdef duc_debug
#include "bits/debug.h"
#else
#define debug(...)
#endif
/* #define int long long */
const int mod = 105;
int n, x, a[mod];
void solve(){
cin >> x >> n;
long long ans = x;
for(int i = 1; i <= n; i++){
cin >> a[i];
ans = ans + x - a[i];
}
cout << ans;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int test = 1;
/* cin >> test; */
for(int i = 1; i <= test; i++){
/* cout << "Case " << "#" << i << ": "; */
solve();
}
return 0;
}
