foo.c: In function 'main':
foo.c:8:16: warning: statement with no effect [-Wunused-value]
  for (i=0;i<n;i+1){
               ~^~
foo.c:13:11: warning: format '%f' expects argument of type 'double', but argument 2 has type 'int' [-Wformat=]
  printf("%f",S);
          ~^
          %d
foo.c:11:9: warning: variable 'A' set but not used [-Wunused-but-set-variable]
  double A=0;
         ^
foo.c:4:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&N);
  ^~~~~~~~~~~~~~
foo.c:9:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&a[i]);
  ^~~~~~~~~~~~~~~~~

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