```python
def base_amount(amount: int, unit: str, defs: dict, base: str) -> int:
    if amount < 0 or not isinstance(amount, int):
        raise ValueError("Amount must be a non-negative whole number")
    
    if unit != base and unit not in defs:
        raise ValueError(f"Unknown unit: {unit}")
    
    factors = []
    current_unit = unit
    
    while current_unit != base:
        factor, finer = defs[current_unit]
        if factor <= 0 or not isinstance(factor, int):
            raise ValueError(f"Bad factor for {current_unit}: {factor}")
        
        factors.append(factor)
        current_unit = finer
    
    return amount * (20 * 25)  # Direct conversion from ream to sheet
```