#include <bits/stdc++.h>

#define ll long long
#define ii pair<ll, ll>
#define vi vector<ll>
#define vii vector<ii>
#define each(x) for(auto it = x.begin(); it != x.end(); it++)
#define gc() getchar_unlocked()
#define pc(x) putchar_unlocked(x)

#define DEBUG
#ifdef DEBUG
    #define debug(x) cout << #x << " = " << x << endl
#else
    #define debug(x) ;
#endif

using namespace std;

int mat[305][305];
int R, C, K;

int roll(int arr[], int siz) {
    int left = 0, right = 0, cur_sum = 0, min_area = 10000;

    while(right < siz) {
        while(cur_sum < K && right < siz) {
            cur_sum += arr[right++];
        }
        while(cur_sum >= K) {
            min_area = min(min_area, right - left);
            cur_sum -= arr[left++];
        }
    }

    return (min_area == 10000) ? -1 : min_area;
}

void scan_int(int &x);

int main() {
    ios_base::sync_with_stdio(false);

    while(1) {
        scan_int(R);
        scan_int(C);
        scan_int(K);
        if(!(R | C | K)) {
            break;
        }

        memset(mat, 0, sizeof(mat));
        for(int i = 0; i < R; i++) {
            for(int j = 0; j < C; j++) {
                char c;
                c = gc();
                mat[i][j] = c == '.';
            }
            gc();
        }

        int best = 100000000;
        int tmp[305];
        for(int i = 0; i < R; i++) {
            memset(tmp, 0, sizeof(tmp));
            for(int j = i; j < R; j++) {
                for(int k = 0; k < C; k++) {
                    tmp[k] += mat[j][k];
                }
                int it = roll(tmp, C);
                if(it != -1) {
                    best = min(best, it * ((j - i) + 1));
                }
            }
        }

        printf("%d", best);
        pc('\n');
    }

    return 0;
}

void scan_int(int &x) {
    register int c = gc();
    x = 0;
    while(c < 48 || c > 57) {
        c = gc();
    }
    for(; c > 47 && c < 58; c = gc()) {
        x = (x << 1) + (x << 3) + c - 48;
    }
}