#include <bits/stdc++.h>
using namespace std;
ifstream f("fermier.in");
ofstream g("fermier.out");
int v[4002], M, N, P, i, aux, ok, G, x, y;
int main()
{
    f>>N>>M>>P;
    aux=P;
    for (i=1;i<=P;i++)
    {
        f>>x>>y;
        v[i]=(x-1)*M+y;
    }
    sort (v+1, v+P+1);
    v[0]=G;
    v[P+1]=1ULL*M*N+1;
    for (i=1;i<=P+1;i++)
    {
        if (v[i]-v[i-1]>1)
        if (ok==0)
        {
            G=v[i]-v[i-1]-1+G;
            ok=1;
        }
        else ok=0;
    }
    g<<G;
    return 0;
}