public class Solution {
    /*
     * @param A: the given number
     * @param B: another number
     * @return: the last digit of B! / A!
     */
    public int computeLastDigit(long A, long B) {
        if(A == B) return 1;
        long left = A+1;
        long right = B;
        int prod = 1;
        for(long i=left;i<=right;i++){
            int lastDigit = (int)i%10;
            prod = (prod * lastDigit)%10;
            if(prod == 0) return 0;
        }
        return prod;
    }
}