#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);++i)
#define ALL(A) A.begin(), A.end()

using namespace std;

typedef long long ll;
typedef pair<int, int> P;

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int N, M; cin >> N >> M;
	vector<int> a(M, 0);
	rep (i, M) cin >> a[i];

	int res = N - 1 - M;
	cout << res << endl;

	return 0;
}