#include <stdio.h>

int main(){
	long long type = 0;
	scanf("%lld",&type);
	
	long long ans = 0;
	for(long long i = 0;i < type;i++){
		long long chair,table;
		scanf("%lld%lld",&chair,&table);
		
		ans += (chair / 2) * table;
 	}
	printf("%lld",ans);
}