#include <iostream>

using namespace std;

int main() {
    int n;
    cin >> n;
    int x, y;
    int cnt = 0;
    int arrnr[100][100] = {0,};
    for (int i = 0;i < n;i++) {
        cin >> x >> y;
         for (int j = x;j < 10+x;j++){
            for (int k =y;k < 10+y;k++){
                arrnr[j][k] = 1;
            }
        }
    }
    
    for (int i =  0;i < 100 ;i++ ) {
        for (int j = 0;j < 100 ;j++ ) {
            cnt += arrnr[i][j];
            
        }
    }
    cout << cnt;
    return 0;
}