The data I have is as described below:

Input:
======

```
df = pd.DataFrame({"col1": ["A1", "A2"], "col2": ["B1", "B2"], "2015-1": [231, 432], "2015-2": [456, 324]})
print(df)
  col1 col2  2015-1  2015-2
0   A1   B1     231     456
1   A2   B2     432     324

```

The columns `2015-1` and `2015-2` corresponds to years and months. The data I want to transform is as below:

Output:
=======

```
print(df)
  col1 col2  year    month  values
0   A1   B1  2015        1     231
1   A1   B1  2015        2     456
2   A2   B2  2015        1     432
3   A2   B2  2015        2     324

```

I want to Transform the data into another dataframe without constructing a loop since the data I have contains lots of columns and rows, it takes a really long time. Is there any way to convert the input into output without a loop?