#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 d[2001][2001][26];
int k;
bool check1(int h,int w,int sa){
    int cnt=0;
    rep(i,0,26){
        int b=d[h+sa][w+sa][i]-d[h+sa][w][i]-d[h][w+sa][i]+d[h][w][i];
        if(b>0)cnt++;
    }
    return cnt>=k;
}
bool check2(int h,int w,int sa){
    int cnt=0;
    rep(i,0,26){
        int b=d[h+sa][w+sa][i]-d[h+sa][w][i]-d[h][w+sa][i]+d[h][w][i];
        if(b>0)cnt++;
    }
    return cnt>k;
}
int h,w;
int solve(int nh,int nw){
    int l=0,r1=0,r2=0;
    while(nh+r1<=h&&nw+r1<=w){
        if(check1(nh,nw,r1-l))break;
        r1++;
        r2++;
    }
    while(nh+r2<=h&&nw+r2<=w){
        if(check2(nh,nw,r2-l))break;
        r2++;
    }
    int res=r2-r1;
    for(;;){
        if(r1+nh>h||r1+nw>w)break;
        l++;
        while(nh+r1<=h&&nw+r1<=w){
            if(check1(nh+l,nw+l,r1-l))break;
            r1++;
            
        }
        while(nh+r2<=h&&nw+r2<=w){
            if(check2(nh+l,nw+l,r2-l))break;
            r2++;
        }
        res+=r2-r1;
    }
    return res;
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin>>h>>w>>k;
    string s[h];rep(i,0,h)cin>>s[i];
   
    rep(i,0,h+1){
        rep(l,0,26)d[i][0][l]=0;
        
    }
    rep(i,0,w+1){
        rep(l,0,26)d[0][i][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){
        ans+=solve(i,0);
       // cout<<solve(i,0);
    }
    rep(j,1,w){
        ans+=solve(0,j);
       // cout<<solve(0,j);
    }
    cout<<ans<<endl;
    
}