#include <iostream>
#include <sstream>
#include <cstring>
#include <algorithm>
#include <string>
#include <vector>
#include <set>

using namespace std;

class FoxSequence {
public:
string isValid(vector <int> seq) {
    int a, b, c, d;

    // between 0 and a
    for(int i = 0; i < seq.size(); i++) {
        if(seq[i] >= seq[i+1]) { a = i; break; }
    }
    // between a and b
    for(int i = a; i < seq.size(); i++) {
        if(seq[i] <= seq[i+1]) { b = i; break; }
    }
    // between b and c
    for(int i = b; i < seq.size(); i++) {
        if(seq[i] != seq[i+1]) { c = i; break; }
    }
    // between c and d
    for(int i = c; i < seq.size(); i++) {
        if(seq[i] >= seq[i+1]) { d = i; break; }
    }

    if(a <= 0) return "NO";
    int diff = (seq[a] - seq[0]) / a;
    for(int i = 0; i < a - 1; i++) {
       if(seq[i+1] - seq[i] != diff) {
           return "NO";
        }
    }

    diff = (seq[b] - seq[a]) / (b - a);
    for(int i = a; i < b - 1; i++) {
       if(seq[i+1] - seq[i] != diff) {
           return "NO";
        }
    }

    diff = 0;
    for(int i = b; i < c - 1; i++) {
       if(seq[i+1] - seq[i] != diff) {
           return "NO";
        }
    }

    diff = (seq[d] - seq[c]) / (d - c);
    for(int i = c; i < d - 1; i++) {
       if(seq[i+1] - seq[i] != diff) {
           return "NO";
        }
    }
    int N = seq.size();
    diff = (seq[N - 1] - seq[d]) / ((N - 1) - d);
    for(int i = d; i < (N - 1) - 1; i++) {
       if(seq[i+1] - seq[i] != diff) {
           return "NO";
        }
    }

    return "YES";
}
};