complete 10419.cpp and cpp Makefile update

storage/zeta/cpp/completed/10419.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+
+int max_late_time(int d) {
+    int s = 1;
+    while (s * s + s <= d) {
+        s++;
+    }
+    return s - 1;
+}
+
+int main() {
+    int T;
+    std::cin >> T;
+    int d;
+    for (int t = 0; t < T; t++) {
+        std::cin >> d;
+        std::cout << max_late_time(d) << std::endl;
+    }
+    return 0;
+}