module top_module( 
    input [399:0] a, b,
    input ci,
    output reg co,
    output reg [399:0] sum );
    
    reg carry;
    reg carry_temp;
    integer i;
    always @ (*)
    begin
        carry = 0;
        for (i = 0; i< 100; i = i + 1)
        begin
            if(i == 0)  {carry_temp,sum[i*4 +: 4]} = a[i*4 +: 4] + b[i*4 +: 4]  + ci;
            else        {carry_temp,sum[i*4 +: 4]} = a[i*4 +: 4] + b[i*4 +: 4]  + carry;
                
            if({carry_temp,sum[i*4 +: 4]} >= 10)
            begin
                sum[i*4 +: 4] = {carry_temp,sum[i*4 +: 4]} - 10;
                carry = 1;
            end
            else carry = 0;
        end
        co = carry;
    end
endmodule