#include<bits/stdc++.h>
using namespace std; 
const int MOD=1e9 +7 ; 
 
char bar[1005][1005]; 
int dp[3][2005]; 
int white[1005]={0}; 
int sum[1005];
int main()
{
 
	int n,m,x,y; 
	cin>>n>>m>>x>>y; 
 
	for(int i=0;i<n;i++){
		for(int j=0;j<m;j++){
			cin>>bar[i][j];
			if(bar[i][j]=='.') white[j]++;   
		}
	}
 
 
 dp[0][0]=0; 
 dp[1][0]=0; 
 
for(int i=0;i<m;i++) sum[i+1]=sum[i]+white[i];
 
memset(dp,INT_MAX ,sizeof(dp)); 
 
for(int i=0;i<m;i++){
	for(int j=x;j<=y;j++){
		if(i+j>m){break;}
 
		dp[0][i+j]=min(dp[0][i+j],dp[1][i]+sum[i+j]-sum[i]);
		dp[1][i+j]=min(dp[1][i+j],dp[0][i]+(j)*n-(sum[i+j]-sum[i])); 
	}	
}
 
 
cout<<min(dp[0][m],dp[1][m])<<endl;;
 
return 0;   
}

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