Как выследить ошибку?

#include<iostream>
int F (int L,G){return(L+G);}
 
int main ()
{   int a,b,c;  cin>>a;  cin>>b;      
     c= F ( a,b); cout << c;system("pause");
  return(0);
}

int F (int L, int G)

#include<iostream>
using namespace std;
 
int main ()
{   
     cout << "int" <<siseof (int) <<" bites";
     cout << "short" <<siseof (short) <<"bites";
     cout << "char" <<siseof (char) <<"bites";
     cout << "float" <<siseof (float) <<"bites";
     cout << "double" <<siseof (double) <<"bites";
     cout << "bool" <<siseof (bool) <<"bites";
  return(0);
}

#include<iostream>
using namespace std;
 
int main ()
{   
     cout << "int  " <<sizeof (int) <<" bites\n";
     cout << "short  " <<sizeof (short) <<" bites\n";
     cout << "char  " <<sizeof (char) <<" bites\n";
     cout << "float  " <<sizeof (float) <<" bites\n";
     cout << "double  " <<sizeof (double) <<" bites\n";
     cout << "bool  " <<sizeof (bool) <<" bites\n";
     system("pause  ");
     cout << " (unsigned)int = " << sizeof(int) << endl;
    cout << " (unsigned)short = " << sizeof(short) << endl;
    cout << " (unsigned)char = " << sizeof(char) << endl;
    cout << " (unsigned)float = " << sizeof(float) << endl;
    cout << " (unsigned)double = " << sizeof(double) << endl;
    cout << " (unsigned)long = " << sizeof(long) << endl;
    cout << " (unsigned)long double = " << sizeof(long double) << endl;
    system("pause  ");
     for( int i=0 ; i<=128 ; i++)
         cout << i  << "  "<<  (char)i  << endl ;
     system("pause  ");
  return(0);
}

#include<iostream>
using namespace std;
 
int tsifra;
 
inline int Func (int peremm)
{   return  peremm*2;    }
 
 
int main ()
{   
   int i;
  cin<< tsifra <<i;
 
  for( ;i>=0;i--){
      tsifra=Func ( peremm); cout << " tsifra = " << tsifra << endl;}
 
 system("pause  ");
  return(0);
}