#include<bits/stdc++.h>
using namespace std;
long long n,m,a,b,c[25][25],dp[25][25];
int main(){
	cin>>n>>m>>a>>b;
	c[a][b]=-1;
	c[a+2][b+1]=-1;
	c[a+1][b+2]=-1;
	if(b>=1)c[a+2][b-1]=-1;
	if(b>=2)c[a+2][b-2]=-1;
	if(a>=1)c[a-1][b+2]=-1;
	if(a>=2)c[a-2][b+1]=-1;
	if(b>=1&&a>=2)c[a-2][b-1]=-1;
	if(b>=2&&a>=1)c[a-1][b-2]=-1;
	for(int i=0;i<=m;i++)if(c[0][i]!=-1)dp[0][i]=1;
	for(int i=0;i<=n;i++)if(c[i][0]!=-1)dp[i][0]=1;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=m;j++){
			if(c[i][j]!=-1)dp[i][j]=max(dp[i-1][j],dp[i][j-1])+c[i][j];
			if(c[i][j]==-1)dp[i][j]=0;
		}
	}
	cout<<dp[n][m];
	return 0;
}