#include <cstdio>

using namespace std;

int main(){
  int n;
  unsigned long long ans = 0;
  scanf("%d", &n);
  for(int i = 0; i < n; i++){
    unsigned long long c, d;
    scanf("%llu %llu", &c, &d);
    ans += (unsigned long long)((c + 1) / 2) * d;
  }
  printf("%llu\n", ans);
  return 0;
}