void work(){
	int l,r,mid;
	l = 1; r = n;
	while(l < r){
		mid = (l + r) >> 1;
		if( ok(mid) ) r = mid - 1;else l = mid + 1;
	}
	mid = max(1, mid - 50);
	//cout <<"mid = " << mid <<endl; 
	if(!ok(mid)) ++ mid ;
	printf("%d\n", mid );
}
WA 



void work(){
	int l,r,mid;
	l = 1; r = n;
	while(l < r){
		mid = (l + r) >> 1;
		if( ok(mid) ) r = mid ;else l = mid + 1;
	}
	printf("%d\n", r );
}

AC

我实在不明白为啥，求大牛指点……