A study was conducted in the small town of Fair Falcon to investigate the incidence of a strain of flu virus among 10 people over a 15-year period. This strain of flu-virus can reoccur in the same individual. It is also known that this strain of flu virus lasts 6 months. One person moved away and was unable to be contacted after six years but did not contract the disease during that time; two people went missing after 9.5 years but did not contract the disease during that time. All others were followed up for the full 15 years - during this time: one person contracted the disease at two years, five years and eight years into the study; one person contracted the disease at five and eight years into the study and another one person contracted the disease at 14.5 years into the study. The remaining people did not contract the disease in the study period.

Calculate the number of person-years of at-risk time in this study.