#!/usr/bin/ python3.8
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
import numpy as np

H, W, K = map(int, readline().split())
C = np.frombuffer(read(), 'S1').reshape(H, -1)[:, :W]

INF = np.int32(10 ** 5)


def calc_alphabet(a):
    dp = (C != a) * INF
    for n in range(1, H):
        np.minimum(dp[n - 1] + 1, dp[n], out=dp[n])
    for n in range(1, W):
        np.minimum(dp[:, n - 1] + 1, dp[:, n], out=dp[:, n])
        np.minimum(dp[:-1, n - 1] + 1, dp[1:, n], out=dp[1:, n])
    return dp


dp = np.empty((26, H, W), np.int32)
for i in range(26):
    a = chr(ord('a') + i).encode()
    dp[i] = calc_alphabet(a)


dp.sort(axis=0)
low = dp[K - 1]
if K == 26:
    high = np.full_like(low, INF)
else:
    high = dp[K]
for n in range(min(H, W)):
    np.minimum(high[n, n:], n + 1, out=high[n, n:])
    np.minimum(high[n:, n], n + 1, out=high[n:, n])

x = high - low
answer = np.maximum(x, 0).sum()
print(answer)