The general term is (2r−1)!(27−2r)!1 for r=1 to 13.
Multiply and divide by 26!: (2r−1)!(27−2r)!1=26!1(2r−126)
S=26!1r=1∑13(2r−126)=26!1[(126)+(326)+⋯+(2526)]
Sum of odd-indexed binomial coefficients: ∑(odd26)=225
S=26!225
13S=26!13⋅225=26⋅25!13⋅225=2⋅25!225=25!224
So k=24,n=25, giving n+k=49.