#!/usr/bin/env python3
n,l = [int(i) for i in input().split()]
a = sorted([input() for i in range(n)])
t = 1
p = 0
for i in range(1,n):
    for j in range(l):
        if a[i-1][j] != a[i][j]:
            t += j+1 + max(j-p,0)
            p = j
            break
print(t)