#include<bits/stdc++.h>
using namespace std;
int dx[] = {0, -2, -1, 1, 2, 2, 1, -1, -2};
int dy[] = {0, 1, 2, 2, 1, -1, -2, -2, -1};
int bx,by,cx,cy;
long long f[30][30];
bool s[30][30];
int main(){
cin>>bx>>by>>cx>>cy;
bx+=2;
by+=2;
cx+=2;
cy+=2;
f[2][2] = 1;
s[cx][cy] = 1;
for(int i = 1; i <= 8; i++)s[cx+dx[i]][cy+dy[i]]=1;
for(int i = 2; i <= bx; i++){
for(int j = 2; j <= by; j++){
if(s[i][j]) continue;
f[i][j] = max(f[i][j],f[i-1][j]+f[i][j-1]);
}
}
cout<<f[bx][by];
return 0;
}
