Here is a Python solution for the problem:

```python
def evaluate_expression(expression):
stack = []
for token in expression.split():
if token in ["+", "-", "*", "/"]:
operand2 = stack.pop()
operand1 = stack.pop()
if token == "+":
result = operand1 + operand2
elif token == "-":
result = operand1 - operand2
elif token == "*":
result = operand1 * operand2
elif token == "/":
result = operand1 / operand2
stack.append(result)
else:
try:
stack.append(int(token))
except ValueError:
return -1
return stack[0]

# Test cases
print(evaluate_expression("2 7 +")) # Output: 9
print(evaluate_expression("6 4 - 2 *")) # Output: 4
print(evaluate_expression("1 2 @")) # Output: -1
print(evaluate_expression("2 3 +")) # Output: 5
print(evaluate_expression("5 2 - 3 *")) # Output: 9
```

This solution works by iterating over the tokens in the expression. If a token is an operator, it pops the top two operands from the stack, performs the operation, and pushes the result back onto the stack. If a token is a number, it pushes it onto the stack. If a token is neither an operator nor a number, it returns -1. At the end of the expression, the result is the only item left on the stack.

Question

Here is a Python solution for the problem:

```python
def evaluate_expression(expression):
    stack = []
    for token in expression.split():
        if token in ["+", "-", "*", "/"]:
            operand2 = stack.pop()
            operand1 = stack.pop()
            if token == "+":
                result = operand1 + operand2
            elif token == "-":
                result = operand1 - operand2
            elif token == "*":
                result = operand1 * operand2
            elif token == "/":
                result = operand1 / operand2
            stack.append(result)
        else:
            try:
                stack.append(int(token))
            except ValueError:
                return -1
    return stack[0]

# Test cases
print(evaluate_expression("2 7 +"))  # Output: 9
print(evaluate_expression("6 4 - 2 *"))  # Output: 4
print(evaluate_expression("1 2 @"))  # Output: -1
print(evaluate_expression("2 3 +"))  # Output: 5
print(evaluate_expression("5 2 - 3 *"))  # Output: 9
```

This solution works by iterating over the tokens in the expression. If a token is an operator, it pops the top two operands from the stack, performs the operation, and pushes the result back onto the stack. If a token is a number, it pushes it onto the stack. If a token is neither an operator nor a number, it returns -1. At the end of the expression, the result is the only item left on the stack.

Knowee AI · Accepted Answer

Here is a Python solution for the problem:

```python
def evaluate_expression(expression):
    stack = []
    for token in expression.split():
        if token in ["+", "-", "*", "/"]:
            operand2 = stack.pop()
            operand1 = stack.pop()
            if token == "+":
                result = operand1 + operand2
            elif token == "-":
                result = operand1 - operand2
            elif token == "*":
                result = operand1 * operand2
            elif token == "/":
                result = operand1 / operand2
            stack.append(result)
        else:
            try:
                stack.append(int(token))
            except ValueError:
                return -1
    return stack[0]

# Test cases
print(evaluate_expression("2 7 +"))  # Output: 9
print(evaluate_expression("6 4 - 2 *"))  # Output: 4
print(evaluate_expression("1 2 @"))  # Output: -1
print(evaluate_expression("2 3 +"))  # Output: 5
print(evaluate_expression("5 2 - 3 *"))  # Output: 9
```

This solution works by iterating over the tokens in the expression. If a token is an operator, it pops the top two operands from the stack, performs the operation, and pushes the result back onto the stack. If a token is a number, it pushes it onto the stack. If a token is neither an operator nor a number, it returns -1. At the end of the expression, the result is the only item left on the stack.

Question

Solution

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