import sys

input = sys.stdin.readline

n = int(input())
l = 0
r = 2 * 10**9 + 1
for _ in range(50):
    mid = (l + r) // 2
    if mid * (mid + 1) == 2 * n:
        print("YES")
        exit()
    elif mid * (mid + 1) < 2 * n:
        l = mid
    else:
        r = mid
print("YES" if r * (r + 1) == 2 * n else "NO")