#include <stdio.h>
#include <stdlib.h>

int main() {
	int t, s, n, i, a = 0;

	scanf("%d", &t);

	for (i = 0; i < t; i++) {
		scanf("%d %d", &s, &n);
		a += (s / 2 + s % 2) * n;
	}

	printf("%d\n", a);

	return EXIT_SUCCESS;
}