#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
const long long INF = 1000000007;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
string gh = "here";
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n, m, x, y;
  cin >> n >> m >> x >> y;
  vector<int> v1(m + 1);
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    for (int j = 1; j <= m; j++) {
      if (s[j - 1] == '.') v1[j]++;
    }
  }
  vector<int> h1(m + 1), h2(m + 1), d1(m + 1), d2(m + 1);
  for (int i = 1; i <= m; i++) {
    if (i == 1) {
      d2[i] = v1[i];
    } else {
      d1[i] = d1[i - 1] + v1[i - 1];
      d2[i] = d2[i - 1] + v1[i];
    }
  }
  for (int i = 1; i <= m; i++) {
    v1[i] = n - v1[i];
  }
  for (int i = 1; i <= m; i++) {
    if (i == 1) {
      h2[i] = v1[i];
    } else {
      h1[i] = h1[i - 1] + v1[i - 1];
      h2[i] = h2[i - 1] + v1[i];
    }
  }
  vector<vector<int> > dp(2, vector<int>(m + 1, INF));
  dp[1][0] = dp[0][0] = 0;
  for (int i = 1; i <= m; i++) {
    for (int q = 0; q < 2; q++) {
      for (int a = x; a <= y && a <= i; a++) {
        int sum;
        if (q == 0) {
          sum = d2[i] - d1[i - a + 1];
        } else {
          sum = h2[i] - h1[i - a + 1];
        }
        dp[q][i] = min(dp[q][i], dp[1 - q][i - a] + sum);
      }
    }
  }
  cout << min(dp[0][m], dp[1][m]) << '\n';
  return 0;
}
