import kotlin.math.max

fun main() {
    val (n, l) = readln().split(" ").map(String::toInt)
    var names = (1..n).map { readln() }.sorted()
    names = listOf(" ".repeat(l), *names.toTypedArray(), " ".repeat(l))

    var sum = 0;
    for (j in 1..n) {
        var max = 0
        for (k in 0..1) {
            for (c in 0 until l) {
                if (names[j + k - 1][c] != names[j + k][c]) {
                    max = max(max, c + 1);
                    break;
                }
            }
        }
        sum += max;
    }
    println(sum)
}