请教一下pta中出现的问题

在手机编译器上运行的没问题，但是在pta上却四个测试点都没通过

以下是error描述
输出
a.c: In function ‘main’:
a.c:5:2: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
scanf("%d %d",&N,&M);
^~~~~~~~~~~~~~~~~~~~
a.c:7:9: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
scanf("%d",&a[i]);
^~~~~~~~~~~~~~~~~
a.c:4:2: warning: ‘i’ is used uninitialized in this function [-Wuninitialized]
int N,M,k,i,temp,a[i];
^~~