В программе выходит ошибка: TypeError: 'int' object is not subscriptable

a =  [15, -4, 8, 3 ]
m = 1
n = 4
for i in range(m):
    for j in range(n):
        k = n%(j+1)
        if k == 0:    
            j == 0
        elif k == n-1:
            j-1 == j+n-1
            s = a[i][j+n-1] + a[i][0] + a[i][j] + a[i][j]
        else:
            s = a[i][j-1] + a[i][j+1] + a[i][j] + a[i][j]
            print(s, '', end = '')