#include<stdio.h>
int main(void){
	int n, i;
	long c[10000], d[10000], total = 0;
	
	scanf("%d", &n);
	for(i = 0 ; i < n ; i++){
		scanf("%ld %ld", &c[i], &d[i]);
		if(c[i]%2 == 0){
			c[i] /= 2;
		}else{
			c[i] = (c[i]-1)/2 + 1;
		}
	}
	for(i = 0 ; i < n ; i++){
		total += c[i]*d[i];
	}
	printf("%ld\n", total);	
	return 0;
}