//marico el que lo lea
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <stdlib.h>
#include <assert.h>
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> ii;

void fastIO() {std::ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);}

#define pb push_back
#define all(obj) obj.begin(), obj.end()
#define ms(obj, val) memset(obj, val, sizeof(obj))
#define ms2(obj, val, sz) memset(obj, val, sizeof(obj[0])*sz)

#define fst first
#define snd second

template<typename T, typename U> inline void mnze(T &x, U y) {if(y < x) x = y;}
template<typename T, typename U> inline void mxze(T &x, U y) {if(x < y) x = y;}

void _scan( int &x ) { scanf("%d",&x); }
void _scan( long long &x ) { scanf("%lld",&x); }
void _scan( double &x ) { scanf("%lf",&x); }
void _scan( char &x ) { scanf(" %c",&x); }
void _scan( char *x ) { scanf("%s",x); }
template<typename T>
void _scan( vector<T> &v ){
	for(int i=0; i<(int)v.size(); i++) _scan(v[i]);
}
void scan() {}
template<typename T, typename... U>
void scan( T& head, U&... tail ) { _scan(head); scan(tail...);}

template<typename T> void _dbg(const char* sdbg, T h)
	{ cerr<<sdbg<<"="<<h<<"\n"; }
template<typename T, typename... U> void _dbg(const char* sdbg, T h, U... t) {
	while(*sdbg != ',')cerr<<*sdbg++;
	cerr<<"="<<h<<","; _dbg(sdbg+1, t...);
}

#ifdef LOCAL
#define debug(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
#define debugv(x) {{cerr<<#x<<" = "; for(auto _i:x) cerr<<_i<<", ";cerr<<"\n";}}
#define debuga(x, sz) {{cerr<<#x<<" = ";for(int _i=0;_i<sz;_i++)cerr<<x[_i]<<", ";cerr<<"\n";}}
#else
#define debug(...) (__VA_ARGS__)
#define debugv(x)
#define debuga(x, sz)
#define cerr if(0)cout
#endif

ll MOD = 1e9+7;

inline ll Msum(ll x){return x;}
template<typename... Rest>
inline ll Msum(ll x, Rest... rest){ return (x + Msum(rest...)) % MOD; }

inline ll Mprod(ll x){return x;}
template<typename... Rest>
inline ll Mprod(ll x, Rest... rest){ return x * Mprod(rest...) % MOD; }

inline ll Mnorm(ll x){ return (x%MOD + MOD) %MOD; }
////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////

const int MAXN = 702;

int n, m;
int mat[MAXN][MAXN];
vi G[MAXN];

bool good=true;
int X[2];
vector<ii>  b;

int vis[MAXN];
void dfs(int v, int c){
	vis[v]=c;
	X[c]++;
	for(int u : G[v]){
		if(vis[u]==-1) dfs(u, 1-c);
		else if(vis[u]==vis[v]) good=false;
	}
}

int c2(int x){
	return x*(x-1)/2;
}

int dp[MAXN][MAXN];
int f(int i, int x, int y){
	if(i==(int)b.size()){
		return c2(x)+c2(y);
	}
	int &ret=dp[i][x];
	if(ret != -1) return ret;
	ret = min(f(i+1, x+b[i].fst, y+b[i].snd),
				f(i+1, x+b[i].snd, y+b[i].fst));
	return ret;
}


int main(){
	scan(n, m);
	for(int i=0; i<n; i++) for(int j=0; j<n; j++) mat[i][j] = i!=j;
	for(int i=0; i<m; i++){
		int v, u; scan(v, u); v--; u--; mat[v][u] = mat[u][v] = 0;
	}
	for(int v=0; v<n; v++) for(int u=0; u<n; u++) if(mat[v][u]) G[v].pb(u);
	ms(vis, -1);
	for(int v=0; v<n; v++) if(vis[v]==-1){
		X[0]=X[1]=0;
		dfs(v, 0);
		b.pb({X[0], X[1]});
	}
	int ans;
	ms(dp, -1);
	if(good) ans = f(0, 0, 0);
	else ans = -1;
	printf("%d\n", ans);
}


Measured memory_bytes: 6912