Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| State (class) | Vacator | Reason for change | Successor | Date of successors formal installation |
| --- | --- | --- | --- | --- |
| South Carolina (3) | Olin D. Johnston (D) | Died April 18, 1965 | Donald S. Russell (D) | April 22, 1965 |
| South Carolina (3) | Donald S. Russell (D) | Successor elected November 8, 1965 | Ernest Hollings (D) | November 9, 1965 |
| Virginia (1) | Harry F. Byrd (D) | Resigned November 10, 1965 | Harry F. Byrd, Jr. (D) | November 12, 1965 |
| Michigan (2) | Patrick V. McNamara (D) | Died April 30, 1966 | Robert P. Griffin (R) | May 11, 1966 |
| Virginia (2) | A. Willis Robertson (D) | Resigned December 30, 1966 | William B. Spong, Jr. (D) | December 31, 1966 |
Question:
How many reasons were given when A. Willis Robertson (D) resigned?
Answer:
1.0