#include <stdio.h>

int main(){
	long long type = 0;
	scanf("%lld",&type);
	
	long long num = 0;
	for(long long i = 0;i < type;i ++){
		long long chair,table;
		scanf("%lld%lld",&table,&chair);
		
		if((chair % 2) == 0){
			num += chair / 2;
		}else if((chair % 2) == 1){
			num += (chair - 1) / 2;
		}
	}
	printf("%lld",num);
}