#include <bits/stdc++.h>
//#include <math.h>
using namespace std;

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

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

//-----------------------------------------------------------------------------

#define int long long

int n,m;
int a[100000],b[100000];

struct UnionFind {
	vector<int> data;
	UnionFind(int sz):data(sz,-1) {}
	bool unite(int x,int y) {
		x=root(x),y=root(y);
		if(x!=y) {
			if(data[y]<data[x]) swap(x,y);
			data[x]+=data[y],data[y]=x;
		}
		return x==y;
	}
	bool same(int x,int y) {
		return root(x)==root(y);
	}
	int root(int x) {
		return data[x]<0?x:root(data[x]);
	}
	int size(int x) {
		return -data[root(x)];
	}
};

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin>>n>>m;
	UnionFind uf(n);
	REP(i,m) {
		cin>>a[i]>>b[i];
		a[i]--,b[i]--;
		uf.unite(a[i],b[i]);
	}
	int mi=min(uf.size(0),uf.size(1));
	int ma=n-mi;
	cout<<mi*(mi-1)/2+ma*(ma-1)/2-m<<endl;

	return 0;
}
