Code:

#include <stdio.h>

main()
{
    unsigned n,i,o;
    float Rekv;
    char c;
    
    printf("\n\tProkram racuna ekvivalntnu otpornost n provodnika.\n\n");
    
    printf("\n\tUnesite koliko ima otpornika:");
    scanf("%d",&n);
    
    printf("\n\tOPCIJA:");
    printf("\n\tr-redna veza\n\n\tp-paralelna veza");
    
    printf("\n\n\tVas izbor je:");
    
    do{
        printf("\n\t       ");
        scanf("%c",&c);
        
    }while(c!='r'&&c!='R'&&c!='p'&&c!='P');
    
    printf("\n\tUnesite vrednost otpornika u omima:");
    
    switch(c)
        {
            case 'r': case 'R':
                Rekv=0;
                for(i=0;i<n;i++)
                    {
                        printf("\n\tR%d=",i+1);
                        do{
                            scanf("%d",&o);
                        }while(o<=0);
                        Rekv+=o;
                    }
            break;
            
            case 'p': case 'P':
                Rekv=0;
                for(i=0;i<n;i++)
                    {
                        printf("\n\tR%d=",i+1);
                        do{
                            scanf("%d",&o);
                        }while(o<=0);
                        Rekv+=1/o;
                    }
                Rekv=1/Rekv;
            break;
        }
        
    printf("\n\tRekv=%.3f",Rekv);
    
    printf("\n\n\n\n\n");
}

Code:

 case 'p': case 'P': for(i=0;i<n;i++){
             printf("\n\tR%d=",i+1);
             scanf("%d",&o);
             if(i==0)
                 Rekv=o;
             else
                         Rekv=(Rekv*o)/(Rekv+o);
            }break;