Croatian Highschool 2008, National Competition September 23, 2013

#include <cstdio>
#include <algorithm>
#include <vector>
#include <set>
#include <cmath>
using namespace std;
#define pi 3.1415926535
const double sec1=sqrt(3)/4-pi/12;
const double sec2=1-(pi/4);
const double sec3=pi;
const double sec4=pi/6+sqrt(3)/4;
const double sec5=pi/4;
bool a[1111][1111];
  int main() {
    int X,Y;
    scanf("%d%d",&X,&Y);
    int n;
    scanf("%d",&n);
    for (int i = 0; i < n; i++) {
        int nx,ny;
        scanf("%d%d",&nx,&ny);
        a[nx][ny]=true;
    }
    double ans=0;
    for (int x1 = 0; x1 < X; x1++)
        for (int y1 = 0; y1 < Y; y1++) {
            int x2=x1+1, y2=y1+1;
            if ((a[x1][y1] && a[x2][y2]) || (a[x1][y2] && a[x2][y1])) ans++;
            else if (a[x1][y1]+a[x1][y2]+a[x2][y1]+a[x2][y2] == 2) ans += sec4;
            else if (a[x1][y1]+a[x1][y2]+a[x2][y1]+a[x2][y2] == 1) ans += sec5;
        }
    printf("%.2lf\n",ans);
    return 0;
}