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

using ll = int;
using ull = unsigned long long;
using ld = long double;
using pll = pair<ll, ll>;
using vll = vector<ll>;
using vpll = vector<pll>;

#define pb push_back
#define FOR(i, m, n) for (ll i(m); i < n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define F(n) REP(i, n)
#define FF(n) REP(j, n)
#define FFF(n) REP(k, n)
#define D(x) cerr << "D(" << #x << "): " << x << endl
#define EPS (1e-10)
#define INF (1LL<<61)
#define CL(A, I) memset(A, I, sizeof(A))
#define all(x) begin(x), end(x)
#define IN(n) ll n; cin >> n;
#define x first
#define y second
void ga(ll N, ll* A){F(N) cin >> A[i];}

const int MX = 1000007;
ll ft[MX];
ll get (ll en) {
    ll sum=0;
    for(;en;en-=en&-en) sum+=ft[en];
    return sum;
}
ll get(int st, int en) {
    return get(en)-get(st-1);
}
void inc(int i, ll val) {
    for(;i<MX;i+=i&-i)ft[i]+=val;
}

ll binary(ll s,ll e,ll val){
    if(e==s)return e;
    ll m = (s+e)/2;
    if(get(1,m)>=val) 
        return binary(s,m,val);
    else return binary(m+1,e,val);
}

int main(void)
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    while(1){
        ll M,N;
        cin>>M>>N;
        if(!cin)break;
        ll K=N/2;
        vector<pll> a;
        F(N) {
            ll x,y; cin>> x>>y; a.pb({x,y}); 
        }
        long long ret=INF;
        FFF(4){
            for(auto& i:a) 
                i={M-i.y-1,i.x};
            
            CL(ft,0);
            set<ll>xs,ys;
            unordered_map<ll,ll> mx,my;
            unordered_map<ll,ll> mx2,my2;
            for(auto &i: a) {
                xs.insert(i.x);ys.insert(i.y);
            }
            ll ii=1; for(auto &i:xs) { mx[ii]=i; mx2[i]=ii++;}
            ii=1; for(auto &i:ys) { my[ii]=i; my2[i]=ii++; }
            sort(a.begin(),a.end());
            vector<vll> cols(mx.size()+1);
            for(auto &i:a) {
                cols[mx2[i.x]].pb(my2[i.y]);
            }
            ll r=0;
            for(auto &v:cols) {
                for(auto &t:v) inc(t,1);
                ll a=binary(1,MX-1,K);
                //D(mx[r]);D(my[a]);D(K);D(get(1,a+1));
                if(get(1,a)!=K){r++;continue;}
                long long s = (mx[r]+1)*(1LL+my[a]);
                // D(s);
                if (ret > s) {
                    ret=s;
                }
                r++;
            }
        }
        cout<<(ret==INF||N%2 ? -1 : ret)<<endl;
    }
    return 0;
}