但凡不是用来读取的变量记得赋值初始化，double变量在输入时使用lf，控制一位小数点用"%.1f"

  #include <iostream>
  #include <cstdio>

  using namespace std;

  int main(){
      int n,s = 0;
      double gs = 0;
      scanf("%d",&n);
      for(int i = 0; i < n ; i++){
          double ai;
          int ci;
          scanf("%lf%d",&ai,&ci);
          gs += ai * ci;
          s += ci;
      }
      double GPA = gs / s;
      printf("%.1f",GPA);
      return 0;
  }