some update

zeta_python/completed/19939.py

@@ -0,0 +1,13 @@
+def solve(N, K):
+    Kp = (K * (K + 1)) // 2
+    if N < Kp:
+        return -1
+    else:
+        N -= Kp
+        r = N % K
+        return sum((K - 1, 1 if r > 0 else 0))
+
+
+if __name__ == '__main__':
+    N, K = map(int, input().split())
+    print(solve(N, K))