#include <iostream>
#include <vector>
#include <string>
#include <sstream>

using namespace std;

class FoxSequence {
public:
	string isValid(vector <int> seq) {
		int n = seq.size();
		bool ok = false;
		for(int a=1;a<n;a++){
			bool check = seq[1]-seq[0]>0;
			for(int i=1;i+1<=a;i++) check &= (seq[1]-seq[0] == seq[i+1]-seq[i]);
			if(!check) break;
			for(int b=a+1;b<n;b++){
				check = seq[a+1]-seq[a]<0;
				for(int i=a+1;i+1<=b;i++) check &= (seq[a+1]-seq[a] == seq[i+1]-seq[i]);
				if(!check) break;
				for(int c=b;c<n;c++){
					check = true;
					for(int i=b+1;i<=c;i++) check &= (seq[i] == seq[b]);
					if(!check) break;
					for(int d=c+1;d+1<n;d++){
						check = seq[c+1]-seq[c]>0;
						for(int i=c+1;i+1<=d;i++) check &= (seq[c+1]-seq[c] == seq[i+1]-seq[i]);
						if(!check) continue;
						check = seq[d+1]-seq[d]<0;
						for(int i=d+1;i+1<n;i++) check &= (seq[d+1]-seq[d] == seq[i+1]-seq[i]);
						if(!check) continue;
						if(check) ok = true;
					}
				}
			}
		}
		return ok ? "YES" : "NO";
	}
// BEGIN CUT HERE
	public:
	void run_test(int Case) { if ((Case == -1) || (Case == 0)) test_case_0(); if ((Case == -1) || (Case == 1)) test_case_1(); if ((Case == -1) || (Case == 2)) test_case_2(); if ((Case == -1) || (Case == 3)) test_case_3(); if ((Case == -1) || (Case == 4)) test_case_4(); if ((Case == -1) || (Case == 5)) test_case_5(); }
	private:
	template <typename T> string print_array(const vector<T> &V) { ostringstream os; os << "{ "; for (typename vector<T>::const_iterator iter = V.begin(); iter != V.end(); ++iter) os << '\"' << *iter << "\","; os << " }"; return os.str(); }
	void verify_case(int Case, const string &Expected, const string &Received) { cerr << "Test Case #" << Case << "..."; if (Expected == Received) cerr << "PASSED" << endl; else { cerr << "FAILED" << endl; cerr << "\tExpected: \"" << Expected << '\"' << endl; cerr << "\tReceived: \"" << Received << '\"' << endl; } }
	void test_case_0() { int Arr0[] = {1,3,5,7,5,3,1,1,1,3,5,7,5,3,1}
; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); string Arg1 = "YES"; verify_case(0, Arg1, isValid(Arg0)); }
	void test_case_1() { int Arr0[] = {1,2,3,4,5,4,3,2,2,2,3,4,5,6,4}
; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); string Arg1 = "YES"; verify_case(1, Arg1, isValid(Arg0)); }
	void test_case_2() { int Arr0[] = {3,6,9,1,9,5,1}
; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); string Arg1 = "YES"; verify_case(2, Arg1, isValid(Arg0)); }
	void test_case_3() { int Arr0[] = {1,2,3,2,1,2,3,2,1,2,3,2,1}
; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); string Arg1 = "NO"; verify_case(3, Arg1, isValid(Arg0)); }
	void test_case_4() { int Arr0[] = {1,3,4,3,1,1,1,1,3,4,3,1}
; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); string Arg1 = "NO"; verify_case(4, Arg1, isValid(Arg0)); }
	void test_case_5() { int Arr0[] = {6,1,6}
; vector <int> Arg0(Arr0, Arr0 + (sizeof(Arr0) / sizeof(Arr0[0]))); string Arg1 = "NO"; verify_case(5, Arg1, isValid(Arg0)); }

// END CUT HERE
};

// BEGIN CUT HERE
int main() {
	FoxSequence ___test;
	___test.run_test(-1);
}
// END CUT HERE