#include <stdio.h>
int globina (int * a, int * b) {
	int globina[2];
	if (*a)
		globina[0] = globina(a+*a, b);
	if (*b)
		globina[1] = globina(a, b+*b);
	
}
int main (void) {
	int n;
	scanf("%d\n", &n);
	int a[n];
	int b[n];
	for (int i = 0; i < n; i++)
		scanf("%d %d\n", &a[i], &b[i]);
	printf("%d\n", globina(a, b));
}