#include<bits/stdc++.h>
//ios::sync_with_stdio(false);
//cin.tie(0);
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<pii,int> ppii;
typedef pair<int,pii> pipi;
typedef pair<ll,ll> pll;
typedef pair<pll,ll> ppll;
typedef pair<ll,pll> plpl;
typedef tuple<ll,ll,ll> tl;
ll mod=1000000007;
ll mod2=998244353;
ll mod3=1000003;
ll inf=1000000000000000000;
double pi=2*acos(0);
#define rep(i,m,n) for(int i=m;i<n;i++)
#define rrep(i,n,m) for(int i=n;i>=m;i--)
int dh[4]={1,-1,0,0};
int dw[4]={0,0,1,-1};
int ddh[8]={-1,-1,-1,0,0,1,1,1};
int ddw[8]={-1,0,1,-1,1,-1,0,1};
ll lmax(ll a,ll b){
    if(a<b)return b;
    else return a;
}
ll lmin(ll a,ll b){
    if(a<b)return a;
    else return b;
}
ll gcd(ll a,ll b){
    if(a<b)swap(a,b);
    if(a%b==0)return b;
    return gcd(b,a%b);
}
ll Pow(ll n,ll k){
    ll ret=1;
    ll now=n;
    while(k>0){
        if(k&1)ret*=now;
        now*=now;
        k/=2;
    }
    return ret;
}


int main(){
    
    int h,w,k;cin>>h>>w>>k;
    string s[h];rep(i,0,h)cin>>s[i];
    int d[h+1][w+1][26];
    rep(i,0,h+1){
        rep(j,0,w+1)rep(l,0,26)d[i][j][l]=0;
    }
    rep(i,1,h+1){
        rep(j,1,w+1){
            rep(l,0,26){
                d[i][j][l]=d[i-1][j][l]+d[i][j-1][l]-d[i-1][j-1][l];
            }
            d[i][j][s[i-1][j-1]-'a']++;
        }
    }
    ll ans=0;
    rep(i,0,h){
        rep(j,0,w){
            int u=min(h-i,w-j)+1;
            int dd=0;
            while(u-dd>1){
                int mid=(u+dd)/2;
                int cnt=0;
                rep(l,0,26){
                    int b=d[i+mid][j+mid][l]-d[i+mid][j][l]-d[i][j+mid][l]+d[i][j][l];
                    if(b>0)cnt++;
                }
                if(cnt<=k)dd=mid;
                else u=mid;
            }
            int cc=dd;
            u=min(h-i,w-j)+1;
            dd=0;
            while(u-dd>1){
                ll mid=(u+dd)/2;
                int cnt=0;
                rep(l,0,26){
                    int b=d[i+mid][j+mid][l]-d[i+mid][j][l]-d[i][j+mid][l]+d[i][j][l];
                    if(b>0)cnt++;
                }
                if(cnt<k)dd=mid;
                else u=mid;
            }
            ans+=cc-u+1;
            
        }
    }
    printf("%d\n",ans);
}