#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define R cin>>
#define Z class
#define ll long long
#define ln cout<<'\n'
#define in(a) insert(a)
#define pb(a) push_back(a)
#define pd(a) printf("%.10f\n",a)
#define mem(a) memset(a,0,sizeof(a))
#define all(c) (c).begin(),(c).end()
#define iter(c) __typeof((c).begin())
#define rrep(i,n) for(ll i=(ll)(n)-1;i>=0;i--)
#define REP(i,m,n) for(ll i=(ll)(m);i<(ll)(n);i++)
#define rep(i,n) REP(i,0,n)
#define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++)
template<Z A>void pr(A a){cout<<a;ln;}
template<Z A,Z B>void pr(A a,B b){cout<<a<<' ';pr(b);}
template<Z A,Z B,Z C>void pr(A a,B b,C c){cout<<a<<' ';pr(b,c);}
template<Z A,Z B,Z C,Z D>void pr(A a,B b,C c,D d){cout<<a<<' ';pr(b,c,d);}
template<Z A>void PR(A a,ll n){rep(i,n){if(i)cout<<' ';cout<<a[i];}ln;}
ll check(ll n,ll m,ll x,ll y){return x>=0&&x<n&&y>=0&&y<m;}
const ll MAX=1e9+7,MAXL=1LL<<61,dx[4]={-1,0,1,0},dy[4]={0,1,0,-1};
typedef pair<ll,ll> P;

int d[26][2111][2111];
string s[2222];

void init(ll n,ll m) {
  rep(k,26) {
    rep(i,n+1)rep(j,m) d[k][i][j+1]+=d[k][i][j];
    rep(j,m+1)rep(i,n) d[k][i+1][j]+=d[k][i][j];
  }
}
int calc(ll x1,ll y1,ll x2,ll y2,ll k) {return d[k][x2][y2]-d[k][x1][y2]-d[k][x2][y1]+d[k][x1][y1];}

void Main() {
  ll n,m,t;
  cin >> n >> m >> t;
  rep(i,n) cin >> s[i];
  rep(i,n)rep(j,m) d[s[i][j]-'a'][i+1][j+1]++;
  init(n,m);
  ll ans=0;
  for(ll i=0; i<n+m-1; i++) {
    ll x=max(0LL,i-m+1),y=max(0LL,n-i-1),x2=x,y2=y,x3=x,y3=y;
    while(check(n,m,x,y)) {
      x++;
      y++;
      while(x2<x&&y2<y) {
        ll c=0;
        rep(k,26) {
          ll z=calc(x2,y2,x,y,k);
          if(z) c++;
        }
        if(c<t) break;
        x2++;
        y2++;
      }
      while(x3<x&&y3<y) {
        ll c=0;
        rep(k,26) {
          ll z=calc(x3,y3,x,y,k);
          if(z) c++;
        }
        if(c<t+1) break;
        x3++;
        y3++;
      }
      ans+=x2-x3;
    }
  }
  pr(ans);
}

int main(){ios::sync_with_stdio(0);cin.tie(0);Main();return 0;}