## https://yukicoder.me/problems/no/866

def solve2(h, w, K, l, cum_dp_array):
    h1 = h + l - 1
    w1 = w + l - 1

    answer = 0
    for word in range(26):
        cum_dp = cum_dp_array[word]
        ans = cum_dp[h1 + 1][w1 + 1] - cum_dp[h1 + 1][w] - cum_dp[h][w1 +1] + cum_dp[h][w]
        if ans > 0:
            answer += 1
    return answer <= K

def solve(H, W, K, cum_dp_array):
    answer = 0
    for h in range(H):
        for w in range(W):
            low = 1
            high = min( (H - h), (W - w))
            while high - low > 1:
                mid = (high + low) // 2
                if solve2(h, w, K, mid, cum_dp_array):
                    low = mid
                else:
                    high = mid
            if solve2(h, w, K, high, cum_dp_array):
                answer += high
            else:
                answer += low
    return answer


def main():
    H, W, K = map(int, input().split())
    C = []
    for _ in range(H):
        C.append(input())

    cum_dp_array = []
    for word in range(26):
        dp = [[0] * W for _ in range(H)]
        for h in range(H):
            for w in range(W):
                if ord(C[h][w]) - ord("a") == word:
                    dp[h][w] += 1

        cum_dp = [[0] * (W + 1 ) for _ in range(H + 1)]
        for h in range(H):
            row  = 0
            for w in range(W):
                row += dp[h][w]
                cum_dp[h + 1][w + 1] = cum_dp[h][w + 1] + row
        cum_dp_array.append(cum_dp)

    ans1 = solve(H, W, K, cum_dp_array)        
    if K > 1:
        ans2 = solve(H, W, K - 1, cum_dp_array)
        ans1 -= ans2
    print(ans1)












if __name__ == "__main__":
    main()