[iatra

/// se da o matrice mare a[m,n], stiind ca este completat cu numere 1,2,...,m*n
/// diagonala dupa diagonala, serpuit.
/// Dandu-se linia si coloana, sa se calculeze valoarea lui k=a[lin,col]}
#include <bits/stdc++.h>
#define LL long long int
using namespace std;
ifstream f("piatra.in");
ofstream g("piatra.out");
LL p,minim,m,s,n,lin,col,k;


int main()
{
    f>>m>>n>>lin>>col;
    k=0;
    if(m<n)
        minim=m;
    else
        minim=n;
    if(minim+1>lin+col)
    {
        s=(lin+col-2)*(lin+col-1)/2;
        if((lin+col)%2==1)
            k=s+lin;
        else
            k=s+col;
    }
    else
        if(m+n-minim+1<lin+col)
        {
            p=m+n-lin-col;
            s=m*n-p*(p+1)/2;
            if((lin+col)%2==1)
                k=s-m+lin;
            else
                k=s-n+col;
        }
        else
        {
            s=minim*(minim+1)/2;
            p=col+lin-minim-2;
            s=s+p*minim;
            if(n<m)
                if((lin+col)%2==1)
                    k=s+n-col+1;
                else
                    k=s+col;
            else
                if((lin+col)%2==1)
                    k=s+lin;
                else
                    k=s+m-lin+1;
        }
    g<<k;
    f.close();
    g.close();
    return 0;
}