#include <bits/stdc++.h>
#define rep(i, n) for (lli i = 0; i < (n); i++)
#define rrep(i, n) for (lli i = (n)-1; i >= 0; i--)
using namespace std;

using lli = long long int;
void YESNO(bool), YesNo(bool);
template <class T1, class T2>
bool chmin(T1 &l, const T2 &r);
template <class T1, class T2>
bool chmax(T1 &l, const T2 &r);
int c[2005][2005];
int sum[26][2005][2005] = {};
lli f(int l, int i, int j, int p)
{
    return sum[l][i + p][j + p] - sum[l][i + p][j] - sum[l][i][j + p] + sum[l][i][j];
}
lli h,
    w, k;
lli calc(int i, int j, int p)
{
    // [up, n)でp種類以上
    lli up = min(h - i, w - j) + 1;
    lli low = 0;
    while (up - low > 1)
    {
        lli mid = (up + low) / 2;
        lli cnt = 0;
        rep(l, 26)
            cnt += (f(l, i, j, mid) >= 1);
        (cnt >= p ? up : low) = mid;
    }
    return up;
}
int main()
{
    cin >> h >> w >> k;
    rep(i, h) rep(j, w)
    {
        char ch;
        cin >> ch;
        c[i][j] = ch - 'a';
    }
    rep(l, 26)
    {
        rep(i, h) rep(j, w)
            sum[l][i + 1][j + 1] = sum[l][i][j + 1] + sum[l][i + 1][j] - sum[l][i][j] + int(c[i][j] == l);
    }
    lli ret = 0;
    rep(i, h) rep(j, w)
    {
        // cout << i << " " << j << " " << calc(i, j, k) << endl;
        // cout << i << " " << j << " " << calc(i, j, k + 1) << endl;
        ret += calc(i, j, k + 1) - calc(i, j, k);
    }
    cout << ret << endl;
}

// -- lib
void YESNO(bool b) { cout << (b ? "YES" : "NO") << endl; }
void YesNo(bool b) { cout << (b ? "Yes" : "No") << endl; }

template <class T1, class T2>
bool chmin(T1 &l, const T2 &r)
{
    return (l > r) ? (l = r, true) : false;
}

template <class T1, class T2>
bool chmax(T1 &l, const T2 &r)
{
    return (l < r) ? (l = r, true) : false;
}