vector<ll> v[1d5+1];

ll bs[1d5];
ll ad,as[1d5+1];

{
	int @n,@m;
	rd(bs(n));
	rep(m){
		ll @l,@r;
		v[l-1].push_back(r);
	}
	rep(i,0,n,2)bs[i]=-bs[i];
	rep(x,n){
		ad+=as[x];
		if(v[x].empty()){
			if(bs[x]!=ad){
				wt("NO");
				exit(0);
			}
			continue;
		}
		int r=v[x][0];
		rep(y,1,v[x].size()){
			int r2=v[x][y];
			if(r>r2) swap(r,r2);
			v[r].push_back(r2);
		}
		v[x].clear();
		as[r]-=bs[x]-ad;
		ad=bs[x];
	}
	wt("YES");
}