the "if" statement accounts for those values by subtracting a number from the sum if it is to the left of a larger number

for instance...

MCMXC = [1000, 100, 1000, 10, 100]

if(data[numbers[i]] < data[numbers[i+1])... this is triggered when data[numbers[1]] AKA 100 is less than data[numbers[1+1]] AKA 1000

so...sum += data[numbers[1+1]] - data[numbers[1]] AKA 1000 - 100