def solve(n, A):
    if A.count(A[2]) != 1:
        return False
    if n <= 1:
        return False
    if A[0] == A[5]:
        return False
    return True
    

n = int(input())
A = list(map(int, input().split()))

if solve(n, A):
    print("YES")
else:
    print("NO")