set.seed(1)
df <- data.frame( year = c(seq(2000, 2018), seq(2000, 2018)) , id = c(rep(1, 19),rep(2, 19)), varA = floor(rnorm(38)*100), varB= floor(rnorm(38)*100), varC= floor(rnorm(38)*100))

df <- df %>% mutate(varA = if_else(year>2010, as.double(NA) , varA) ,
                    varB = if_else(year>2010, as.double(NA) , varB),
                    varC = if_else(year>2010, as.double(NA) , varC))  %>% group_by(id) %>% arrange(year)

df <- df %>% mutate( varD = lag(varA)*lag(varB),
                     varRESULT = if_else(is.na(varC), lag(varC, 1)/lag(varD, 2)*lag(varD, 1), varC))