#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes.by_ref().map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr,) => {};
    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => { ($(read_value!($next, $t)),*) };
    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };
    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };
    ($next:expr, usize1) => (read_value!($next, usize) - 1);
    ($next:expr, [ $t:tt ]) => {{
        let len = read_value!($next, usize);
        read_value!($next, [$t; len])
    }};
    ($next:expr, $t:ty) => ($next().parse::<$t>().expect("Parse error"));
}

trait Change { fn chmax(&mut self, x: Self); fn chmin(&mut self, x: Self); }
impl<T: PartialOrd> Change for T {
    fn chmax(&mut self, x: T) { if *self < x { *self = x; } }
    fn chmin(&mut self, x: T) { if *self > x { *self = x; } }
}

fn main() {
    input! {
        h: usize, w: usize, k: usize,
        c: [chars; h],
    }
    let mut acc = vec![vec![[0; 26]; w + 1]; h + 1];
    for u in 0..26 {
        for i in 0..h {
            for j in 0..w {
                acc[i + 1][j + 1][u] = acc[i + 1][j][u] + acc[i][j + 1][u]
                    - acc[i][j][u] + if c[i][j] as u8 - b'a' == u as u8 {
                        1
                    } else {
                        0
                    };
            }
        }
    }
    let mut tot = 0;
    for i in 0..h {
        for j in 0..w {
            let mut pass = 0;
            let mut fail = min(w - j, h - i) + 1;
            while fail - pass > 1 {
                let mid = (fail + pass) / 2;
                let mut ans = 0;
                for u in 0..26 {
                    if acc[i + mid][j + mid][u] - acc[i][j + mid][u]
                        - acc[i + mid][j][u] + acc[i][j][u] > 0 {
                            ans += 1;
                        }
                }
                if ans < k {
                    pass = mid;
                } else {
                    fail = mid;
                }
            }
            let lb = fail;
            let mut fail = min(w - j, h - i) + 1;
            while fail - pass > 1 {
                let mid = (fail + pass) / 2;
                let mut ans = 0;
                for u in 0..26 {
                    if acc[i + mid][j + mid][u] - acc[i][j + mid][u]
                        - acc[i + mid][j][u] + acc[i][j][u] > 0 {
                            ans += 1;
                        }
                }
                if ans <= k {
                    pass = mid;
                } else {
                    fail = mid;
                }
            }
            let ub = fail;
            if lb < ub {
                tot += (ub - lb) as i64;
            }
        }
    }
    println!("{}", tot);
}