import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
public class Main {
public static void main(String[] args) throws Exception {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
int count = Integer.parseInt(br.readLine());
boolean[][] location = new boolean[101][101];
int x;
int y;
int area = 0;
for ( int i = 0 ; i < count ; i ++) {
StringTokenizer st = new StringTokenizer(br.readLine());
x = Integer.parseInt(st.nextToken());
y = Integer.parseInt(st.nextToken());
for(int X = x ; X < x+10 ; X++) {
for(int Y = y ; Y < y+10 ; Y++) {
if(location[X][Y] == false) {
location[X][Y] = true;
area++;
}
}
}
}
System.out.print(area);
}
}