const int N=200099;
int n,q,tr[N<<2]={},fa[N<<1]={};
ll ans=0;
int father(int x) { return fa[x]=fa[x]==x?x:father(fa[x]); }
void link(int x,int y) { return fa[father(y)]=father(x),void(); }
bool same(int x,int y) { return father(x)==father(y); }
struct edge {
	int l,r; ll c;
	friend bool operator < (const edge a,const edge b) {
		return a.c<b.c;
	}
} e[N]={};
#define lson (x<<1)
#define rson (x<<1|1)
void update(int x) {
	tr[lson]=father(tr[lson]),tr[rson]=father(tr[rson]);
	if(!tr[lson]||!tr[rson]) tr[x]=0;
	else if(same(tr[lson],tr[rson])) tr[x]=father(tr[lson]);
	else tr[x]=0;
	return ;
}
void setup(int x,int l,int r) {
	if(l==r) return tr[x]=l,void();
	int mid=l+r>>1;
	setup(lson,l,mid),setup(rson,mid+1,r);
	return update(x);
}
void link(int x,int l,int r,int p,ll cost) {
	if(tr[x]) {
		if(!same(tr[x],p))
			link(tr[x],p),ans+=cost;
		if(l==r) tr[x]=father(l); else update(x);
		return ;
	}
	if(l==r) return ;
	int mid=l+r>>1;
	link(lson,l,mid,p,cost);
	link(rson,mid+1,r,p,cost);
	update(x);
	return ;
}
void change(int x,int l,int r,int L,int R,int p,ll cost) {
	if(L<=l&&r<=R) return link(x,l,r,p,cost);
	int mid=l+r>>1;
	if(L<=mid) change(lson,l,mid,L,R,p,cost);
	if(mid<R) change(rson,mid+1,r,L,R,p,cost);
	update(x);
	return ;
}
int main()
{
//	usefile("mst");
	int i;
	read(n,q);
	for(i=1;i<=q;++i)
		read(e[i].l,e[i].r,e[i].c);
	for(i=1;i<=n+q;++i)
		fa[i]=i;
	sort(e+1,e+1+q);
	setup(1,1,n);
	for(i=1;i<=q;++i)
		change(1,1,n,e[i].l,e[i].r,n+i,e[i].c);
	for(i=1;i<=n+q;++i)
		if(!same(i,1)) {
			printf("-1\n");
			return 0;
		}
	printf("%lld\n",ans);
	return 0;
}