#include <bits/stdc++.h>
using namespace std;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
template<typename T> bool mmax(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template<typename T> bool mmin(T &m, const T q) { if (m > q) {m = q; return true;} else return false; }


using lint = long long int;
int dp[2001][2001][27];
int H, W, K;

constexpr int INF = 100000;
int main()
{
    cin >> H >> W >> K;
    REP(i, H + 1) REP(j, W + 1) REP(d, 27) dp[i][j][d] = INF;
    REP(i, H) REP(j, W)
    {
        char c;
        cin >> c;
        dp[i][j][c - 'a'] = 1;
    }
    IREP(i, H) IREP(j, W) REP(d, 26)
    {
        mmin(dp[i][j][d], min({dp[i + 1][j][d], dp[i][j + 1][d], dp[i + 1][j + 1][d]}) + 1);
    }
    lint ret = 0;
    REP(i, H) REP(j, W)
    {
        vector<int> v(27);
        REP(d, 27) v[d] = dp[i][j][d];
        sort(v.begin(), v.end());
        int d_max = min(H - i, W - j);
        ret += min(v[K] - 1, d_max) - min(v[K - 1] - 1, d_max);
    }
    cout << ret << endl;
}