#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;

//template
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(a);i>(b);i--)
#define ALL(v) (v).begin(),(v).end()
typedef long long int ll; typedef pair<ll, ll> P;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<typename A,size_t N,typename T>void Fill(A(&array)[N],const T &val){fill((T*)array, (T*)(array+N), val);}
const int inf=INT_MAX/2; const ll INF=LLONG_MAX/2;
//template end

int h,w,k; int imos[2001][2001][26]={};
int rui(int x1,int y1,int x2,int y2){
    int res=0;
    rep(i,0,26)if(imos[x2][y2][i]-imos[x1][y2][i]-imos[x2][y1][i]+imos[x1][y1][i]!=0)res++;
    return res;
}
int search(int x,int y,int a){
    int l=-1,r=min(h-x,w-y)+1,mid;
    while(r-l>1){
        mid=(l+r)/2;
        if(rui(x,y,x+mid,y+mid)>=a)r=mid;
        else l=mid;
    }
    return r;
}

int main(){
    scanf("%d%d%d",&h,&w,&k);
    rep(i,0,h)rep(j,0,w){
        char c; cin>>c;
        imos[i+1][j+1][c-'a']++;
    }
    rep(i,0,h)rep(j,0,w+1)rep(l,0,26)imos[i+1][j][l]+=imos[i][j][l];
    rep(i,0,h+1)rep(j,0,w)rep(l,0,26)imos[i][j+1][l]+=imos[i][j][l];
    ll ans=0,add;
    rep(i,0,h)rep(j,0,w){
        add=search(i,j,k+1)-search(i,j,k);
        ans+=add;
    }
    printf("%lld\n",ans);
    return 0;
}