Language: C

m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}

Compiler output:
main.c:1:1: warning: data definition has no type or storage class
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      | ^
main.c:1:1: error: type defaults to 'int' in declaration of 'm' [-Wimplicit-int]
main.c:1:3: error: type defaults to 'int' in declaration of 'f' [-Wimplicit-int]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |   ^
main.c:1:5: error: type defaults to 'int' in declaration of 'r' [-Wimplicit-int]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |     ^
main.c:1:7: error: return type defaults to 'int' [-Wimplicit-int]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |       ^~~~
main.c: In function 'main':
main.c:1:7: error: type of 's' defaults to 'int' [-Wimplicit-int]
main.c:1:21: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |                     ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
main.c:1:21: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |                     ^~~~~
main.c:1:21: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:63: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |                                                               ^~~~~~
main.c:1:63: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:63: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:1:63: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:137: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |                                                                                                                                         ^~~~
main.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
  +++ |+#include <stdlib.h>
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
main.c:1:137: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |                                                                                                                                         ^~~~
main.c:1:137: note: include '<stdlib.h>' or provide a declaration of 'exit'
main.c:1:144: error: expected ';' before '}' token
    1 | m,f,r;main(s){for(;~scanf("%d%d%d",&m,&f,&r)&&m+f!=-2;){s=m+f;printf("%c\n",m*f<0?70:s>79?65:s>64?66:s>49?67:s>29&&r>49?67:s>29?68:70);}exit(0)}
      |                                                                                                                                                ^
      |                                                                                                                                                ;