public static int digitMatch(int a, int b){
    if(a < 0 || b < 0){
        throw new IllegalArgumentException();
    }
    if(a / 10 == 0 || b / 10 == 0){
        if(a % 10 == b % 10){
            return 1;
        }
        return 0;
    }
    if(a % 10 == b % 10){
        return 1 + digitMatch(a / 10, b / 10);
    }
    return digitMatch(a / 10, b / 10);
}