C++ :

#include<bits/stdc++.h>
using namespace std;
int gcd(int a,int b){return b==0?a:gcd(b,a%b);}
int s,b,i,n,m,p;
int main(){
	scanf("%d%d%d",&n,&m,&p);
	b=gcd(n,m)+gcd(abs(p-n),m)+p;
	s=(p*m)/2;
	i=s-b/2+1;
	printf("%d",i);
}

Pascal :

begin end.