n=input("Введите n:")
matrix=[]
summa=0
for i in range(int(n)):
    matrix.append([])
    for j in range(int(n)):
        matrix[i].append(input("Введите "+str(j)+" элемент "+str(i)+" строки:"))
for i in range(int(n)):
    summa+=int(matrix[i][i])
print(summa)

 >>> matrix = [[1,2,3], [4,5,6], [7,8,9]]
>>> [ x[n] for n, x in enumerate(matrix) ]
[1, 5, 9]