foo.cc: In function 'int main()':
foo.cc:14:16: warning: iteration 50 invokes undefined behavior [-Waggressive-loop-optimizations]
   14 |         f[i][j]=1e9;
      |         ~~~~~~~^~~~
foo.cc:12:18: note: within this loop
   12 |     for(int i=0;i<M;i++)
      |                 ~^~

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