#!/usr/bin/env python3
n, l = map(int, input().split())
names = [" " * l, *sorted(input() for _ in range(n)), " " * l]
overlap = lambda a, b: next(i for i, (a, b) in enumerate(zip(a, b)) if a != b) + 1
print(sum(max(overlap(x, y), overlap(y, z)) for x, y, z in zip(names[:-2], names[1:-1], names[2:])))