#ifdef DEBUG
    #define _GLIBCXX_DEBUG
#endif
#pragma GCC optimize("Ofast")
#include <cassert>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <functional>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <complex>
#include <iomanip>
#include <bitset>
#include <random>
#include <chrono>

#define stdin_filepath "stdin.txt"
#define stdout_filepath "stdout.txt"
#define stderr_filepath "stderr.txt"
#define iostream_untie true
#define stdout_precision 10
#define stderr_precision 10
#define rep(i,n) for(int_fast64_t i = 0; i < (int_fast64_t)(n); ++i)
#define all(v) begin(v), end(v)
#define rall(v) rbegin(v), rend(v)
#define fir first
#define sec second
#define u_map unordered_map
#define u_set unordered_set
#define l_bnd lower_bound
#define u_bnd upper_bound
#define emp emplace
#define emf emplace_front
#define emb emplace_back
#define pof pop_front
#define pob pop_back
#define mkp make_pair
#define mkt make_tuple
#define popcnt __builtin_popcountll

using namespace std;
using i64 = int_fast64_t;
using pii = pair<int, int>;
using pll = pair<int_fast64_t, int_fast64_t>;
template <class T> using heap = priority_queue<T>;
template <class T> using minheap = priority_queue<T, vector<T>, greater<T>>;
template <class T> constexpr T inf = numeric_limits<T>::max() / (T)2 - (T)1234567;
constexpr int dx[9] = {1, 0, -1, 0, 1, -1, -1, 1, 0};
constexpr int dy[9] = {0, 1, 0, -1, 1, 1, -1, -1, 0};

namespace execution {
    std::chrono::system_clock::time_point start, end;
    void print_elapsed_time() {
        end = std::chrono::system_clock::now();
        std::cerr << "\n----- Exec time : ";
        std::cerr << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count();
        std::cerr << " ms -----\n";
    }
    struct setupper {
        setupper() {
            if(iostream_untie) {
                ios::sync_with_stdio(false);
                std::cin.tie(nullptr);
                // std::cout.tie(nullptr);
                // std::cerr.tie(nullptr);
            }
            std::cout << std::fixed << std::setprecision(stdout_precision);
            std::cerr << std::fixed << std::setprecision(stderr_precision);
    #ifdef LOCAL
            if(!freopen(stderr_filepath, "wt", stderr)) {
                freopen("CON", "wt", stderr);
                std::cerr << "Failed to open the stderr file\n";
            }
            if(!freopen(stdout_filepath, "wt", stdout)) {
                freopen("CON", "wt", stdout);
                std::cerr << "Failed to open the stdout file\n";
            }
            if(!freopen(stdin_filepath, "rt", stdin)) {
                freopen("CON", "rt", stdin);
                std::cerr << "Failed to open the stdin file\n";
            }
            atexit(print_elapsed_time);
            start = std::chrono::system_clock::now();
    #endif
        }
    } __setupper;
}

namespace std {
    template <class RAitr> void rsort(RAitr __first, RAitr __last) {
        sort(__first, __last, greater<>());
    }
    template <class T> void hash_combine(size_t &seed, T const &key) {
        seed ^= hash<T>()(key) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
    }
    template <class T, class U> struct hash<pair<T,U>> {
        size_t operator()(pair<T,U> const &pr) const
        {
            size_t seed = 0;
            hash_combine(seed,pr.first);
            hash_combine(seed,pr.second);
            return seed;
        }
    };
    template <class tuple_t, size_t index = tuple_size<tuple_t>::value - 1>
    struct hashval_calc {
        static void apply(size_t& seed, tuple_t const& t) {
            hashval_calc<tuple_t, index - 1>::apply(seed, t);
            hash_combine(seed,get<index>(t));
        }
    };
    template <class tuple_t>
    struct hashval_calc<tuple_t, 0> {
        static void apply(size_t& seed, tuple_t const& t) {
            hash_combine(seed,get<0>(t));
        }
    };
    template <class ...T> struct hash<tuple<T...>> {
        size_t operator()(tuple<T...> const& t) const
        {
            size_t seed = 0;
            hashval_calc<tuple<T...>>::apply(seed,t);
            return seed;
        }
    };
}

template <class T, class U> istream &operator>> (istream &s, pair<T,U> &p) { return s >> p.first >> p.second; }
template <class T, class U> ostream &operator<< (ostream &s, const pair<T,U> p) { return s << p.first << " " << p.second; }
template <class T> istream &operator>> (istream &s, vector<T> &v) { for(T &e : v) { s >> e; }   return s; }
template <class T> ostream &operator<< (ostream &s, const vector<T> &v) {
    for(size_t i = 0; i < v.size(); ++i) { s << (i ? " " : "") << v[i]; }   return s;
}
template <class tuple_t, size_t index>
struct tupleos {
    static ostream &apply(ostream &s, const tuple_t &t) {
        tupleos<tuple_t,index - 1>::apply(s,t);
        return s << " " << get<index>(t);
    }
};
template <class tuple_t>
struct tupleos<tuple_t, 0> {
    static ostream &apply(ostream &s, const tuple_t &t) {
        return s << get<0>(t);
    }
};
template <class ...T> ostream &operator<< (ostream &s, const tuple<T...> &t) {
    return tupleos<tuple<T...>, tuple_size<tuple<T...>>::value - 1>::apply(s,t);
}
template <> ostream &operator<< (ostream &s, const tuple<> &t) { return s; }
#ifdef DEBUG
    #define dump(...) cerr << " [ " << __LINE__ << " : " << __FUNCTION__ << " ] " << #__VA_ARGS__ << " : ", dump_func(__VA_ARGS__)
#else
    #define dump(...)
#endif
template <class T> void dump_func(const T &x) { cerr << x << '\n'; }
template <class T,class ...Rest> void dump_func(const T &x, Rest ... rest) { cerr << x << ","; dump_func(rest...); }
template <class T> void write(const T &x) { cout << x << '\n'; }
template <class T, class ...Rest> void write(const T &x, Rest ... rest) { cout << x << ' '; write(rest...); }
void writeln() {}
template <class T, class ...Rest> void writeln(const T &x, Rest ... rest) { cout << x << '\n'; writeln(rest...); }
#define esc(...) writeln(__VA_ARGS__), exit(0)
template <class P> void read(P __first, P __second) { for(P i = __first; i != __second; ++i) cin >> *i; }

template <class T> bool chmin(T &x, const T &y) { return x > y ? x = y, true : false; }
template <class T> bool chmax(T &x, const T &y) { return x < y ? x = y, true : false; }
template <class T> constexpr T minf(const T &x, const T &y) { return min(x,y); }
template <class T> constexpr T maxf(const T &x, const T &y) { return max(x,y); }
constexpr bool odd(int_fast64_t n) { return n & 1; }
constexpr bool even(int_fast64_t n) { return (int)odd(n) ^ 1; }
constexpr bool bit(int_fast64_t n, int e) { return (n >> e) & 1; }
constexpr int_fast64_t mask(int_fast64_t n, int e) { return n & ((1 << e) - 1); }
constexpr int_fast64_t ilog(int_fast64_t x, int_fast64_t b = 2) { return x ? 1 + ilog(x / b, b) : -1; }
constexpr int_fast64_t gcd(int_fast64_t x, int_fast64_t y) {
    x = x > 0 ? x : -x, y = y > 0 ? y : -y;
    while(y) x %= y, x ^= y ^= x ^= y;
    return x;
}
constexpr int_fast64_t lcm(int_fast64_t x, int_fast64_t y) { return x ? x / gcd(x, y) * y : 0; }
int_fast64_t binry(int_fast64_t ok, int_fast64_t ng, const function<bool(int_fast64_t)> &fn) {
    while (abs(ok - ng) > 1) {
        int_fast64_t mid = (ok + ng) / 2;
        (fn(mid) ? ok : ng) = mid;
    }
    return ok;
}
template <class A, size_t N, class T> void init(A (&array)[N], const T &val) { fill((T*)array, (T*)(array + N), val); }
template <class A, size_t N> void init(A (&array)[N]) { memset(array, 0, sizeof(array)); }
template <class T> vector<int> cmprs(const vector<T> &v) {
    vector<T> tmp = v; vector<int> ret;
    sort(begin(tmp),end(tmp));
    tmp.erase(unique(begin(tmp),end(tmp)), end(tmp));
    for(const T &i : v) ret.emplace_back(lower_bound(begin(tmp),end(tmp),i) - begin(tmp));
    return ret;
}
template <class T> vector<int> cmprs(const T *__first, const T *__last) {
    return cmprs(vector<T>(__first, __last));
}
void for_subset(int_fast64_t s, const function<void(int_fast64_t)> &fn) {
    int_fast64_t t = s;
    do { fn(t); } while((--t &= s) != s);
}


/* The main code follows. */


signed main()
{
    void solve();
    void input();
    void init();

    int t = 1;

    #ifdef LOCAL
        t = 3;
        // cin >> t;
    #endif

    // cin >> t;

    while(t--)
    {
        init();
        input();
        solve();
    }
}

int h,w,k;
int g[26][2005][2005];


void init()
{
    init(g);
}

void input()
{
    cin>>h>>w>>k;
    char c;
    for(int i=0; i<h; i++)
    {
        for(int j=0; w>j; j++)
        {
            cin>>c;
            g[c-'a'][i+1][j+1]=1;
        }
    }
}

void solve()
{
    for(int i=0; i<h; i++)
    {
        for(int j=1; j<=w; j++)
        {
            for(int z=0; z<26; z++)
            {
                g[z][i+1][j]+=g[z][i][j];
            }
        }
    }
    for(int i=1; i<=h; i++)
    {
        for(int j=0; j<w; j++)
        {
            for(int z=0; z<26; z++)
            {
                g[z][i][1+j]+=g[z][i][j];
            }
        }
    }

    int ckin(int,int,int,int);
    int prvl[2002],prvu[2002],nowl[2002],nowu[2002];
    init(prvl), init(prvu), init(nowl), init(nowu);

    i64 ans=0;
    for(int i=1; i<=h; i++)
    {
        for(int j=1; w>=j; j++)
        {
            int &x=nowl[j];
            x=prvl[j-1]+1;
            while(ckin(i-x,j-x,i,j)>=k)
            {
                x--;
            }
            int &y=nowu[j];
            y=prvu[j-1]+1;
            while(ckin(i-y,j-y,i,j)>k)
            {
                y--;
            }
            ans+=y-x;
            dump(i,j,x,y);
        }
        swap(nowl,prvl), swap(nowu,prvu);
    }
    write(ans);
}

int ckin(int a,int b,int c,int d)
{
    int ret=0;
    for(int i=0; i<26; i++)
    {
        ret+=bool(g[i][c][d]-g[i][c][b]-g[i][a][d]+g[i][a][b]);
    }
    return ret;
}